Major changes are:

- Some types were accidentally documented, even though they couldn't be
accessed by any exported API functions. Those are removed (especially
from encodings.h).

- Some types were not documented where they should. Documentation is
added for them. Members are not necessarily documented separately if names
are self-explanatory.

- @a XXX refers to parameters of the function, it's inappropriate for
highlighting NULL (change to @c)

- As per consensus, build_info is removed from GeanyData (replaced by
pointer to avoid ABI break; added grep-able abi-todo tag so it doesn't get
forgotten)