Update where documentation (#15467)

# Description

Updates `help where` to better explain row conditions, and provide more
examples. Also, the syntax shape is changed to `one_of(condition,
closure())>`. I don't think this should affect parsing at all because it
should still always be parsed as `SyntaxShape::RowCondition`, but it
should be more clear that you can use a row condition _or_ a closure
here, even if technically we consider closures to be row conditions
internally. In a similar vein, the help text makes this distinction
explicitly to make it more clear to users that closures are supported.

# User-Facing Changes

* Updated `where` help text



---------

Co-authored-by: Bahex <Bahex@users.noreply.github.com>
Co-authored-by: Douglas <32344964+NotTheDr01ds@users.noreply.github.com>
Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-command/src/filters/where_.rs

@@ -10,13 +10,21 @@ impl Command for Where {
     }
 
     fn description(&self) -> &str {
-        "Filter values based on a row condition."
+        "Filter values of an input list based on a condition."
     }
 
     fn extra_description(&self) -> &str {
-        r#"This command works similar to 'filter' but allows extra shorthands for working with
+        r#"A condition is evaluated for each element of the input, and only elements which meet the condition are included in the output.
-tables, known as "row conditions". On the other hand, reading the condition from a variable is
+
-not supported."#
+A condition can be either a "row condition" or a closure. A row condition is a special short-hand syntax to makes accessing fields easier.
+Each element of the input can be accessed through the `$it` variable.
+
+On the left hand side of a row condition, any field name is automatically expanded to use `$it`.
+For example, `where type == dir` is equivalent to `where $it.type == dir`. This expansion does not happen when passing a subexpression or closure to `where`.
+
+When using a closure, the element is passed as an argument and as pipeline input (`$in`) to the closure. Unlike row conditions, the `$it` variable isn't available inside closures.
+
+Row conditions cannot be stored in a variable. To pass a condition with a variable, use a closure instead."#
     }
 
     fn command_type(&self) -> CommandType {
@@ -34,12 +42,12 @@ not supported."#
                 (Type::Range, Type::Any),
             ])
             .required(
-                "row_condition",
+                "condition",
                 SyntaxShape::OneOf(vec![
                     SyntaxShape::RowCondition,
                     SyntaxShape::Closure(Some(vec![SyntaxShape::Any])),
                 ]),
-                "Filter condition.",
+                "Filter row condition or closure.",
             )
             .allow_variants_without_examples(true)
             .category(Category::Filters)
@@ -86,9 +94,9 @@ not supported."#
                 })])),
             },
             Example {
-                description: "Filter items of a list according to a condition",
+                description: "List only the files in the current directory",
-                example: "[1 2] | where {|x| $x > 1}",
+                example: "ls | where type == file",
-                result: Some(Value::test_list(vec![Value::test_int(2)])),
+                result: None,
             },
             Example {
                 description: "List all files in the current directory with sizes greater than 2kb",
@@ -96,13 +104,8 @@ not supported."#
                 result: None,
             },
             Example {
-                description: "List only the files in the current directory",
+                description: r#"List all files with names that contain "Car""#,
-                example: "ls | where type == file",
+                example: r#"ls | where name =~ "Car""#,
-                result: None,
-            },
-            Example {
-                description: "List all files with names that contain \"Car\"",
-                example: "ls | where name =~ \"Car\"",
                 result: None,
             },
             Example {
@@ -110,18 +113,36 @@ not supported."#
                 example: "ls | where modified >= (date now) - 2wk",
                 result: None,
             },
+            Example {
+                description: "Filter items of a list with a row condition",
+                example: "[1 2 3 4 5] | where $it > 2",
+                result: Some(Value::test_list(vec![
+                    Value::test_int(3),
+                    Value::test_int(4),
+                    Value::test_int(5),
+                ])),
+            },
+            Example {
+                description: "Filter items of a list with a closure",
+                example: "[1 2 3 4 5] | where {|x| $x > 2 }",
+                result: Some(Value::test_list(vec![
+                    Value::test_int(3),
+                    Value::test_int(4),
+                    Value::test_int(5),
+                ])),
+            },
             Example {
                 description: "Find files whose filenames don't begin with the correct sequential number",
-                example: "ls | where type == file | sort-by name --natural | enumerate | where {|e| $e.item.name !~ $'^($e.index + 1)' } | each {|| get item }",
+                example: "ls | where type == file | sort-by name --natural | enumerate | where {|e| $e.item.name !~ $'^($e.index + 1)' } | get item",
                 result: None,
             },
             Example {
-                description: r#"Find case-insensitively files called "readme", without an explicit closure"#,
+                description: r#"Find case-insensitively files called "readme", with a subexpression inside the row condition"#,
                 example: "ls | where ($it.name | str downcase) =~ readme",
                 result: None,
             },
             Example {
-                description: "same as above but with regex only",
+                description: r#"Find case-insensitively files called "readme", with regex only"#,
                 example: "ls | where name =~ '(?i)readme'",
                 result: None,
             },