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.
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git-svn-id: https://core.svn.wordpress.org/trunk@25110 1a063a9b-81f0-0310-95a4-ce76da25c4cd