Enforce required, optional, and rest positional arguments start with an uppercase and end with a period. (#11285)

# Description

This updates all the positional arguments (except with
`--features=dataframe` or `--features=extra`) to start with an uppercase
letter and end with a period.

Part of #5066, specifically [this
comment](/nushell/nushell/issues/5066#issuecomment-1421528910)

Some arguments had example data removed from them because it also
appears in the examples.

There are other inconsistencies in positional arguments I noticed while
making the tests pass which I will bring up in #5066.

# User-Facing Changes

Positional arguments are now consistent

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

Automatic documentation updates

crates/nu-command/src/strings/str_/substring.rs

@@ -63,12 +63,12 @@ impl Command for SubCommand {
             .required(
                 "range",
                 SyntaxShape::Any,
-                "the indexes to substring [start end]",
+                "The indexes to substring [start end].",
             )
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
-                "For a data structure input, turn strings at the given cell paths into substrings",
+                "For a data structure input, turn strings at the given cell paths into substrings.",
             )
             .category(Category::Strings)
     }