dm thin: update stale "Status" Documentation

Documentation/device-mapper-/thin-provisioning.txt's "Status" section no
longer reflected the current fitness level of DM thin-provisioning.
That is, DM thinp is no longer "EXPERIMENTAL".  It has since seen
considerable improvement, has been fairly widely deployed and has
performed in a robust manner.

Update Documentation to dispel concern raised by potential DM thinp
users.

Reported-by: Drew Hastings <dhastings@crucialwebhost.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>