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/env/export_env.rs

@@ -19,7 +19,7 @@ impl Command for ExportEnv {
             .required(
                 "block",
                 SyntaxShape::Block,
-                "the block to run to set the environment",
+                "The block to run to set the environment.",
             )
             .category(Category::Env)
     }

crates/nu-command/src/env/load_env.rs

@@ -27,7 +27,7 @@ impl Command for LoadEnv {
             .optional(
                 "update",
                 SyntaxShape::Record(vec![]),
-                "the record to use for updates",
+                "The record to use for updates.",
             )
             .category(Category::FileSystem)
     }

crates/nu-command/src/env/source_env.rs

@@ -24,7 +24,7 @@ impl Command for SourceEnv {
             .required(
                 "filename",
                 SyntaxShape::String, // type is string to avoid automatically canonicalizing the path
-                "the filepath to the script file to source the environment from",
+                "The filepath to the script file to source the environment from.",
             )
             .category(Category::Core)
     }

crates/nu-command/src/env/with_env.rs

@@ -21,12 +21,12 @@ impl Command for WithEnv {
             .required(
                 "variable",
                 SyntaxShape::Any,
-                "the environment variable to temporarily set",
+                "The environment variable to temporarily set.",
             )
             .required(
                 "block",
                 SyntaxShape::Closure(None),
-                "the block to run once the variable is set",
+                "The block to run once the variable is set.",
             )
             .category(Category::Env)
     }