Add some clarifications to the async documentation. Also changed
ASYNC_pause_job() so that it returns success if you are not within the
context of a job. This is so that engines can be used either asynchronously
or synchronously and can treat an error from ASYNC_pause_job() as a real
error.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Document the libssl and command line application aspects of async.
Reviewed-by: NRich Salz <rsalz@openssl.org>