libxl: ao internal API docs: Mention synchronous ao completion

libxl: ao internal API docs: Mention synchronous ao completion

This doc comment about ao lifecycle failed to mention the option of
completing the ao during the initiator function.  (Indeed, the most
obvious reading would forbid it.)

Restructure the comment, describe this situation, and generally
improve the wording.

Also, fix a grammar problem (missing word `a').

Reported-by: Koushik Chakravarty <koushik.chakravarty@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>