Improve documentation of user_can_richedit()

[swissspidy]

The whole docblock for user_can_richedit() needs a bit of an overhaul. Same goes for the user_can_richedit filter.

There are some typos, like WYSIWIG should be WYSIWYG and users browser should be user's browser.

The return value needs to be documented properly, as well as the global variables referenced.

Also, the wording can be improved a bit. For example, Whether the user can access the visual editor. sounds better than Whether the user should have a WYSIWIG editor.

See https://core.trac.wordpress.org/browser/trunk/src/wp-includes/general-template.php?rev=38029#L2875

[gma992]

Clarified descriptions, added $is_edge

[DrewAPicture]

Assigning to mark the good-first-bug as "claimed".

[swissspidy]

Patch only needs some minor tweaks.

[jorbin]

@DrewAPicture - Can you give this a read?

[swissspidy]

Made some minor wording changes.

[swissspidy]

In 38707:

Editor: Improve documentation for user_can_richedit().

Clarifies the various global variables used in the function.

Props gma992.
Fixes #37321.