Clarify inline documentation for object_subtype

[johnbillion]

#35658 introduced the concept (or at least the terminology) of an object subtype when registering meta fields. Reading the inline documentation, it's not at all clear what an object subtype is, or what value should be passed to this parameter.

 * @param string $object_type    The type of object.
 * @param string $object_subtype The subtype of the object type.

Documentation for the $object_subtype parameter for the following functions and filters needs to be improved and made consistent:

• sanitize_meta()
• register_meta()
• registered_meta_key_exists()
• unregister_meta_key()
• get_registered_meta_keys()
• get_registered_metadata()
• sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key} filter
• sanitize_{$object_type}_meta_{$meta_key} filter

[jeremyfelt]

Describing this in a clear way is going to be tough, but we should. :)

WP Core objects are post, user, comment, term.

• Post subtypes are "post", "page", any custom post type slug.
• Term subtypes are "category", "term", any custom taxonomy slug.
• User subtypes are "user". We don't really have custom user types yet.
• Comment subtypes are "comment". We don't really have registered comment types yet.

How do we wrap that up into a sentence or two?

[jeremyfelt]

We can close this ticket out if we move away from object subtypes in #35658.

[jeremyfelt]

The explanation of subtype can wait until another day. :) All instances have been removed in [38095].