First pass at documenting the WP_Filesystem methods. This also introduces stubs of the methods into the base class which are documented, which subclasses can override, some methods were cleaned up at the same time.
See #18476 See #23122. Props kurtpayne, bananastalktome, and, DrewAPicture 

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Improve file header and description in wp-admin/admin-post.php.

Props ericlewis.
Fixes #25362.

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