qapi: Update docs given recent event, spacing fixes

Commit 21cd70df added event support but didn't document what the
generated code looks like.  Commit 05dfb26c removed some unwanted
spaces in the generated code, but didn't reflect those changes
into the documentation.  Finally, the docs start with a big
disclaimer about QMP not using QAPI yet, which feels rather stale.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>