Add some missing information to the inline docs for `get_post_types()`.

* Improves the `@see` reference for `register_post_type()` to mention that it contains information about accepted arguments
* Adds a description of the accepted values for the `$operator` parameter
* Fixes a typo

Fixes #29721.

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git-svn-id: http://core.svn.wordpress.org/trunk@29524 1a063a9b-81f0-0310-95a4-ce76da25c4cd