qapi: Simplify how QAPIDoc implements its state machine

qapi: Simplify how QAPIDoc implements its state machine

QAPIDoc uses a state machine to for processing of documentation lines.
Its state is encoded as an enum QAPIDoc._state (well, as enum-like
class actually, thanks to our infatuation with Python 2).

All we ever do with the state is calling the state's function to
process a line of documentation.  The enum values effectively serve as
handles for the functions.

Eliminate the rather wordy indirection: store the function to call in
QAPIDoc._append_line.  Update and improve comments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190606153803.5278-8-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
[Commit message typo fixed]