Fix operator completion typo and increase consistency (#14006)

# Description

* Fixes missing closing parenthesis on `not-in` completion

Also tweaks the others to give them consistent capitalization and
punctuation:

* First word initial-case, other words lower-case
* Exception - Initial-case for "also known as" after slash or inside
parens
* No closing period for any completion help
* Word-smithing on others.  E.g., "Mod" is technically "Remainder"

# User-Facing Changes

Operator completions

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`
- 
# After Submitting

N/A

crates/nu-cli/src/completions/operator_completions.rs

@@ -40,57 +40,81 @@ impl Completer for OperatorCompletion {
         };
         let possible_operations = match op {
             Expr::Int(_) => vec![
-                ("+", "Plus / Addition"),
-                ("-", "Minus / Subtraction"),
+                ("+", "Add (Plus)"),
+                ("-", "Subtract (Minus)"),
                 ("*", "Multiply"),
                 ("/", "Divide"),
-                ("==", "Equal"),
-                ("!=", "Not Equal"),
-                ("//", "Floor Division"),
-                ("<", "Less Than"),
-                (">", "Greater Than"),
-                ("<=", "Less Than or Equal to"),
-                (">=", "Greater Than or Equal to"),
-                ("mod", "Modulo"),
-                ("**", "Pow"),
-                ("bit-or", "bitwise or"),
-                ("bit-xor", "bitwise exclusive or"),
-                ("bit-and", "bitwise and"),
-                ("bit-shl", "bitwise shift left"),
-                ("bit-shr", "bitwise shift right"),
-            ],
-            Expr::String(_) => vec![
-                ("=~", "Regex Match / Contains"),
-                ("!~", "Not Regex Match / Not Contains"),
-                ("in", "In / Contains (doesn't use regex)"),
+                ("==", "Equal to"),
+                ("!=", "Not equal to"),
+                ("//", "Floor division"),
+                ("<", "Less than"),
+                (">", "Greater than"),
+                ("<=", "Less than or equal to"),
+                (">=", "Greater than or equal to"),
+                ("mod", "Floor division remainder (Modulo)"),
+                ("**", "Power of"),
+                ("bit-or", "Bitwise OR"),
+                ("bit-xor", "Bitwise exclusive OR"),
+                ("bit-and", "Bitwise AND"),
+                ("bit-shl", "Bitwise shift left"),
+                ("bit-shr", "Bitwise shift right"),
+                ("in", "Is a member of (doesn't use regex)"),
+                ("not-in", "Is not a member of (doesn't use regex)"),
                 (
                     "++",
                     "Appends two lists, a list and a value, two strings, or two binary values",
                 ),
-                ("not-in", "Not In / Not Contains (doesn't use regex"),
-                ("starts-with", "Starts With"),
-                ("ends-with", "Ends With"),
+            ],
+            Expr::String(_) => vec![
+                ("=~", "Contains regex match"),
+                ("!~", "Does not contain regex match"),
+                (
+                    "++",
+                    "Appends two lists, a list and a value, two strings, or two binary values",
+                ),
+                ("in", "Is a member of (doesn't use regex)"),
+                ("not-in", "Is not a member of (doesn't use regex)"),
+                ("starts-with", "Starts with"),
+                ("ends-with", "Ends with"),
             ],
             Expr::Float(_) => vec![
-                ("+", "Plus / Addition"),
-                ("-", "Minus / Subtraction"),
+                ("+", "Add (Plus)"),
+                ("-", "Subtract (Minus)"),
                 ("*", "Multiply"),
                 ("/", "Divide"),
-                ("==", "Equal"),
-                ("!=", "Not Equal"),
-                ("//", "Floor Division"),
-                ("<", "Less Than"),
-                (">", "Greater Than"),
-                ("<=", "Less Than or Equal to"),
-                (">=", "Greater Than or Equal to"),
-                ("mod", "Modulo"),
-                ("**", "Pow"),
+                ("==", "Equal to"),
+                ("!=", "Not equal to"),
+                ("//", "Floor division"),
+                ("<", "Less than"),
+                (">", "Greater than"),
+                ("<=", "Less than or equal to"),
+                (">=", "Greater than or equal to"),
+                ("mod", "Floor division remainder (Modulo)"),
+                ("**", "Power of"),
+                ("in", "Is a member of (doesn't use regex)"),
+                ("not-in", "Is not a member of (doesn't use regex)"),
+                (
+                    "++",
+                    "Appends two lists, a list and a value, two strings, or two binary values",
+                ),
             ],
             Expr::Bool(_) => vec![
-                ("and", "Checks if both values are true."),
-                ("or", "Checks if either value is true."),
-                ("xor", "Checks if one value is true and the other is false."),
-                ("not", "Negates a value or expression."),
+                (
+                    "and",
+                    "Both values are true (short-circuits when first value is false)",
+                ),
+                (
+                    "or",
+                    "Either value is true (short-circuits when first value is true)",
+                ),
+                ("xor", "One value is true and the other is false"),
+                ("not", "Negates a value or expression"),
+                ("in", "Is a member of (doesn't use regex)"),
+                ("not-in", "Is not a member of (doesn't use regex)"),
+                (
+                    "++",
+                    "Appends two lists, a list and a value, two strings, or two binary values",
+                ),
             ],
             Expr::FullCellPath(path) => match path.head.expr {
                 Expr::List(_) => vec![(