Document `doc` + `doc-ref` usage

[GreyCat]

Current we only have a very simple doc tag introduction in the first chapter of user guide, and verbose enums section that mentions doc for enum values.

We need to document these properly:

• in user guide
• in reference and/or spec

Cc @julie-bsx, original discussion was in #356.

[GreyCat]

Partially done, at least in the user guide: http://doc.kaitai.io/user_guide.html#ksy-documentation

[GreyCat]

Hopefully, we'll be retiring hand-written text ksy spec soon in favor of JSON schema introduced by @dar2355 in kaitai-io/kaitai_struct_webide#84, so I won't be updating ksy_reference. Closing this one as done.