Move inline docs from register_post_type() to WP_Post_Type::set_props()

[birgire]

The function register_post_type() is a wrapper for a WP_Post_Type object, since version 4.6.

The method WP_Post_Type::set_props() is missing the detailed documentation for each argument, that is present for register_post_type().

So to avoid duplication, it looks like we should move it.

[birgire]

Moved it in 42117.2.diff​

Wasn't sure though, how to handle the @since part, but I moved only the parts that I thought were relevant.