Add a missing `@return` description for `WP_Screen::get_option()`.

Also declare the default value for the `$key` parameter.

Props dustyf for the initial patch.
Fixes #30363.

Built from https://develop.svn.wordpress.org/trunk@30368


git-svn-id: http://core.svn.wordpress.org/trunk@30366 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix parameter description syntax for `WP_Screen::get_option()`.

Props zrothauser for the initial patch.
See [30368]. Fixes #30363.

Built from https://develop.svn.wordpress.org/trunk@30369


git-svn-id: http://core.svn.wordpress.org/trunk@30367 1a063a9b-81f0-0310-95a4-ce76da25c4cd