qapi: Elide "Potential additional modes" from generated docs

qapi: Elide "Potential additional modes" from generated docs

Documentation of BlockExportRemoveMode has

    Potential additional modes to be added in the future:

    hide: Just hide export from new clients, leave existing connections
    as is.  Remove export after all clients are disconnected.

    soft: Hide export from new clients, answer with ESHUTDOWN for all
    further requests from existing clients.

I think this is useful only for developers.  Elide it from generated
documentation by turning it into a TODO section.

This effectively reverts my own commit b71fd73cc45 (Revert "qapi:
BlockExportRemoveMode: move comments to TODO").  At the time, I was
about to elide TODO sections from the generated manual, I wasn't sure
about this one, and decided to avoid change.  And now I've made up my
mind.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240129115008.674248-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>