Add description for 'dayofyear' and 'dayofweek' arguments in WP_Date_Query.

props dziudek.
fixes #25995.
Built from https://develop.svn.wordpress.org/trunk@26234


git-svn-id: http://core.svn.wordpress.org/trunk@26141 1a063a9b-81f0-0310-95a4-ce76da25c4cd