Misc copyedits in docs on built-in types (GH-24466)

# DOC: Improvements in library/stdtypes

This PR does the following:

1. Replaces :meth: by :func: around repr function
2. Adds links to Unicode Standard site
3. Makes explicit "when" you can call the `iskeyword` function. The previous text could cause confusion to readers, especially those with English as a second language. The reader could understand that the `isidentifier` method calls the `iskeyword` function. Now, it is explicit that the dev can do it.
4. Replaces a URL with an inline link.

Automerge-Triggered-By: GH:AlexWaygood