Document more macros

[mejrs]

Currently the following macros appear in the crate root:

• create_exception_type_object
• impl_exception_boilerplate
• pyobject_native_type
• pyobject_native_type_base
• pyobject_native_type_core
• pyobject_native_type_extract
• pyobject_native_type_info
• pyobject_native_type_named
• pyobject_native_type_sized

I'd love to add some documentation/examples for these, but:

• I don't really know what they do
• Which of these are actually meant for users to use? Can the *_native_* macros even be implemented by users? If not should they be private?
• Which ones are implementation details of other macros? Should they be #[doc(hidden)]?

[davidhewitt]

Good idea. TBH I would prefer that we documented none of them for users for now. All of these are implementation-detail in my opinion.

Maybe the pyobject_native_* macros will eventually be useful downstream (e.g. rust-numpy does use a couple of them), however I think that #1308 will require changes to a lot of them so would prefer we didn't encourage people to use them for now.