qapi: Document introduction of gluster's 'debug' option

qapi: Document introduction of gluster's 'debug' option

We intentionally renamed 'debug-level' to 'debug' in the QMP
schema for 'blockdev-add' related to gluster, in order to
match the command line (commit 1a417e46).  However, since
'debug-level' was visible in 2.7, that means that we should
document that 'debug' was not available until 2.8.

The change was intentional because 'blockdev-add' itself
underwent incompatible changes (such as commit 0153d2f) for
the same release; our intent is that after 2.8, these
interfaces will now be stable.  [In hindsight, we should have
used the name x-blockdev-add when we first introduced it]

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20161206182020.25736-1-eblake@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>