Documentation and visibility cleanups in WP_Theme.

 * Declare `__toString()` as public.
 * Use parent() and error() internally, rather than the properties.
 * Add and correct inline documentation throughout.
 * Attempt to clarify that display() is superior to get().
see #20546, see #20103.



git-svn-id: https://core.svn.wordpress.org/trunk@20586 1a063a9b-81f0-0310-95a4-ce76da25c4cd