docs/interop/qmp: Improve OOB documentation

docs/interop/qmp: Improve OOB documentation

OOB documentation is spread over qmp-spec.txt sections 2.2.1
Capabilities and 2.3 Issuing Commands.  The amount of detail is a bit
distracting there.  Move the meat of the matter to new section 2.3.1
Out of band execution.

Throw in a few other improvements while there:

* 2.2 Server Greeting: Drop advice to search entire capabilities
  array; should be obvious.

* 3. QMP Examples

  - 3.1 Server Greeting: Update greeting to the one we expect for the
    release.  Now shows capability "oob".  Update qmp-intro.txt
    likewise.

  - 3.2 Capabilities negotiation: Show client accepting capability
    "oob".

  - 3.7 Out-of-band execution: New.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-4-armbru@redhat.com>
[Whitespace tidied up]