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/removed/format.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
             .optional(
                 "format string",
                 SyntaxShape::String,
-                "the desired date format",
+                "The desired date format.",
             )
             .category(Category::Removed)
     }

crates/nu-command/src/removed/let_env.rs

@@ -14,11 +14,11 @@ impl Command for LetEnv {
         Signature::build(self.name())
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .optional("var_name", SyntaxShape::String, "variable name")
+            .optional("var_name", SyntaxShape::String, "Variable name.")
             .optional(
                 "initial_value",
                 SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
-                "equals sign followed by value",
+                "Equals sign followed by value.",
             )
             .category(Category::Removed)
     }