Add topic on MAVLink C library #defines

[hamishwillee]

Fixes https://github.com/mavlink/mavlink/issues/44

@auturgy @julianoes This needs review - questions inline because most of what I did was guesses.

Note, there are other defines, but I think they are all internal.

[hamishwillee]

@julianoes OK, I've integrated your suggestions, and have one further question: https://github.com/mavlink/mavlink-devguide/pull/492#discussion_r1005011679

If that's correct, is this otherwise OK to go in? The criteria I'd use is that people will have some idea whether they might want to set each of the defines. They might not be certain, but they could either ask or dig for clarification.