Improve inline docs for get_post_types()

[DrewAPicture]

Attached patch fixes a typo in the get_post_types() docs and adds a description for the accepted values of the $operator parameter.

[DrewAPicture]

In 29752:

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.