docs/devel/writing-monitor-commands: Repair a decade of rot

docs/devel/writing-monitor-commands: Repair a decade of rot

The tutorial doesn't match reality since at least 2013.  Repairing it
involves fixing the following issues:

* Update for commit 6d327171551 (aio / timers: Remove alarm timers):
  replace the broken examples.  Instead of having one for returning a
  struct and another for returning a list of structs, do just one for
  the latter.  This resolves the FIXME added in commit
  e218052f928 (aio / timers: De-document -clock) back in 2014.

* Update for commit 895a2a80e0e (qapi: Use 'struct' instead of 'type'
  in schema).

* Update for commit 3313b6124b5 (qapi: add qapi2texi script): add
  required documentation to the schema snippets, and drop section
  "Command Documentation".

* Update for commit a3c45b3e629 (qapi: New special feature flag
  "unstable"): supply the required feature, deemphasize the x- prefix.

* Update for commit dd98234c059 (qapi: introduce x-query-roms QMP
  command): rephrase from "add new command" to "examine existing
  command".

* Update for commit 9492718b7c0 (qapi misc: Elide redundant has_FOO in
  generated C): hello-world's message argument no longer comes with a
  has_message, add a second argument that does.

* Update for moved and renamed files.

While there, update QMP version output to current output.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227115617.237875-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Whitespace tidied up, typo fixed]