Docs: Add missing description for `$weekday_name` parameter and `@return`...

Docs: Add missing description for `$weekday_name` parameter and `@return` entry in `WP_Locale::get_weekday_initial()`.

Props keesiemeijer, 1naveengiri.
Fixes #42793.
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git-svn-id: https://core.svn.wordpress.org/trunk@42190 1a063a9b-81f0-0310-95a4-ce76da25c4cd