Docs: Improve documentation for get_avatar() and related functions.

Includes:
* Expanding and synchronizing documentation for the `default` and `rating` arguments.
* Adding `@since` entries for the `loading`, `fetchpriority`, and `decoding` arguments.
* Wrapping long lines as per the documentation standards.
* Fixing a typo in the `identicon` value.
* Minor formatting edits for consistency.

Follow-up to [31107], [45632], [47554], [53480], [56037].

See #58833.

git-svn-id: https://develop.svn.wordpress.org/trunk@56767 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -1215,13 +1215,13 @@ function rest_add_application_passwords_to_index( $response ) {
 }
 
 /**
- * Retrieves the avatar urls in various sizes.
+ * Retrieves the avatar URLs in various sizes.
  *
  * @since 4.7.0
  *
  * @see get_avatar_url()
  *
- * @param mixed $id_or_email The Gravatar to retrieve a URL for. Accepts a user_id, gravatar md5 hash,
+ * @param mixed $id_or_email The avatar to retrieve a URL for. Accepts a user ID, Gravatar MD5 hash,
  *                           user email, WP_User object, WP_Post object, or WP_Comment object.
  * @return (string|false)[] Avatar URLs keyed by size. Each value can be a URL string or boolean false.
  */