Clarify the Documentation in win_is_writable() and move an inline comment to...

Clarify the Documentation in win_is_writable() and move an inline comment to the Docblock, reduces confusion about what the function actually does. See #22900


git-svn-id: https://core.svn.wordpress.org/trunk@23254 1a063a9b-81f0-0310-95a4-ce76da25c4cd