Docs: Add missing descriptions and `@return` tags for...

Docs: Add missing descriptions and `@return` tags for `_WP_Editors::get_mce_locale()` and `::get_baseurl()`.

Props adnan.limdi.
Fixes #41869.
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git-svn-id: https://core.svn.wordpress.org/trunk@46823 1a063a9b-81f0-0310-95a4-ce76da25c4cd