Sync inline docs between register_post_type() and register_taxonomy()

[ocean90]

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

[ocean90]

In 25130:

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.