Improve documentation for each command (#15172)

# Description

It is a rework of https://github.com/nushell/nushell.github.io/pull/1819

So, I was wasting time looking for equivalent of `filter_map` in Nu,
unaware that `each` already has it. This PR is to make it clear in the
documentation, saving other user's time.

# User-Facing Changes

No

# Tests + Formatting

No

# After Submitting

No

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

@@ -20,7 +20,7 @@ iterate over each record, not necessarily each cell within it.
 
 Avoid passing single records to this command. Since a record is a
 one-row structure, 'each' will only run once, behaving similar to 'do'.
-To iterate over a record's values, try converting it to a table
+To iterate over a record's values, use 'items' or try converting it to a table
 with 'transpose' first."#
     }
 
@@ -70,7 +70,7 @@ with 'transpose' first."#
             },
             Example {
                 example: r#"[1 2 3 2] | each {|e| if $e == 2 { "two" } }"#,
-                description: "Produce a list that has \"two\" for each 2 in the input",
+                description: "'null' items will be dropped from the result list. It has the same effect as 'filter_map' in other languages.",
                 result: Some(Value::test_list(vec![
                     Value::test_string("two"),
                     Value::test_string("two"),