Docs: Link to `add_theme_support()` for the list of supported theme features...

Docs: Link to `add_theme_support()` for the list of supported theme features in `current_theme_supports()` and related functions.

Props stevenlinx.
Fixes #50657.
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git-svn-id: https://core.svn.wordpress.org/trunk@48252 1a063a9b-81f0-0310-95a4-ce76da25c4cd