Add equiv handling for types and callbacks for Erlang

[garazdawi]

This PR adds equiv handling for types and callbacks to the Erlang mode by adding doc_fallback support to the retriever for types and callbacks.

Also sneaking in a doc fix to describe keys available in metadata in groups_for_* functions.

[josevalim]

Apologies in case I am missing something obvious, but can source_doc ever be nil? However, even if nil, doc_ast would return nil anyway, so I think we can skip the source_doc && bit. I would also add a small function, doc_fallback(callback_data) for the second part, so we end-up wit this:

Suggested change

	(source_doc && doc_ast(content_type, source_doc, file: doc_file, line: doc_line + 1)) ||
	(callback_data[:doc_fallback] && callback_data.doc_fallback.())
	doc_ast(content_type, source_doc, file: doc_file, line: doc_line + 1) ||
	doc_fallback(callback_data)

WDYT?

[garazdawi]

Apologies in case I am missing something obvious, but can source_doc ever be nil?

I was just doing the same thing as for function_data, might very well be that source_doc cannot be nil.

Refactor pushed.

[josevalim]

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