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/encode_decode/decode.rs

@@ -25,7 +25,7 @@ impl Command for Decode {
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("decode")
             .input_output_types(vec![(Type::Binary, Type::String)])
-            .optional("encoding", SyntaxShape::String, "the text encoding to use")
+            .optional("encoding", SyntaxShape::String, "The text encoding to use.")
             .category(Category::Strings)
     }
 

crates/nu-command/src/strings/encode_decode/decode_base64.rs

@@ -44,7 +44,7 @@ impl Command for DecodeBase64 {
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
-                "For a data structure input, decode data at the given cell paths",
+                "For a data structure input, decode data at the given cell paths.",
             )
             .category(Category::Hash)
     }

crates/nu-command/src/strings/encode_decode/encode.rs

@@ -29,7 +29,7 @@ impl Command for Encode {
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("encode")
             .input_output_types(vec![(Type::String, Type::Binary)])
-            .required("encoding", SyntaxShape::String, "the text encoding to use")
+            .required("encoding", SyntaxShape::String, "The text encoding to use.")
             .switch(
                 "ignore-errors",
                 "when a character isn't in the given encoding, replace with a HTML entity (like `🎈`)",

crates/nu-command/src/strings/encode_decode/encode_base64.rs

@@ -45,7 +45,7 @@ impl Command for EncodeBase64 {
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
-                "For a data structure input, encode data at the given cell paths",
+                "For a data structure input, encode data at the given cell paths.",
             )
             .category(Category::Hash)
     }