Improve inline docs for register_post_type() and register_taxonomy().

* register_taxonomy: Use the same doc format for the associative array arg as used for register_post_type()
* register_taxonomy: Improve docs for _builtin, capabilities, hierarchical and rewrite args
* register_taxonomy: Use the same order in $defaults as in docblock
* register_taxonomy: Replace is_null with null ===, to be consistent
* register_post_type: Use the same order in $defaults as in docblock
* register_post_type: Improve docs for @uses and default fallbacks
And while we're on it: Whitespaces.

fixes #25150.
Built from https://develop.svn.wordpress.org/trunk@25130


git-svn-id: http://core.svn.wordpress.org/trunk@25110 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Press This: Add admin_head-like hooks that are specific to the page.

props sillybean.
fixes #18843.

Built from https://develop.svn.wordpress.org/trunk@25131


git-svn-id: http://core.svn.wordpress.org/trunk@25111 1a063a9b-81f0-0310-95a4-ce76da25c4cd