mon: make MonMap error message about unspecified monitors less specific.

The error message helpfully references the -m and -c CLI options for
specifying monitors, but this code can be invoked from non-core librados
client applications so that's unfortunately not kosher. Remove the
reference.

Fixes #5979.

Signed-off-by: Greg Farnum <greg@inktank.com>
librados: synchronous commands should return on commit instead of ack

This is unlikely to be noticed by anybody, but it is a big change. Document
in the PendingReleaseNotes and bump up the librados minor version number
to 68.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #512 from ceph/wip-5988

Reviewed-by: Sage Weil <sage@inktank.com>