Mark $single parameter of get_post_meta() as optional.

props netweb.
fixes #28708.
Built from https://develop.svn.wordpress.org/trunk@28954


git-svn-id: http://core.svn.wordpress.org/trunk@28750 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Specify the `$single` parameter default for `get_post_meta()`. Docs spacing.

See #28708.

Built from https://develop.svn.wordpress.org/trunk@28955


git-svn-id: http://core.svn.wordpress.org/trunk@28751 1a063a9b-81f0-0310-95a4-ce76da25c4cd