Docs: Add missing `@return` description for some theme functions:

* `get_stylesheet_directory_uri()`
* `get_stylesheet_uri()`
* `get_locale_stylesheet_uri()`

Props stevenlinx.
Fixes #50450.
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git-svn-id: https://core.svn.wordpress.org/trunk@47907 1a063a9b-81f0-0310-95a4-ce76da25c4cd