ADIv5: doxygen

Provide doxygen for many of the public ADIv5 interfaces (i.e. the ones
called from Cortex core support code).

Add FIXMEs (and a TODO) to help resolve implementation issues which
became more apparent when trying to document this code:

 - Error-prone context-sensitivity (queued/nonqueued) in many procedures.

 - Procedures that lie by ignoring errors and wrongly claiming success.

Also, there was no point in a return from dap_ap_select(); it can't fail,
and no caller checks its return status.  Clean that up, make it void.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>