Convert default arguments documentation for `get_posts()` into a hash notation.

Props coffee2code for the initial patch.
See #28841.

Built from https://develop.svn.wordpress.org/trunk@29113


git-svn-id: http://core.svn.wordpress.org/trunk@28899 1a063a9b-81f0-0310-95a4-ce76da25c4cd