docs/qapi-domain: add :ifcond: directive option

docs/qapi-domain: add :ifcond: directive option

Add a special :ifcond: option that allows us to annotate the
definition-level conditionals.

The syntax of the argument is currently undefined, but it's possible we
can apply better formatting in the future. Currently, we just display
the ifcond string as preformatted text.

Signed-off-by: Harmonie Snow <harmonie@gmail.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-26-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>