docs: Lift restriction on running API from the event loop thread

docs: Lift restriction on running API from the event loop thread

Since v6.2.0-rc1~238 (and friends) QMP processing was moved to a
per-domain thread. Therefore, it is now safe to call APIs from
the event loop thread (e.g. just like qemu shim is doing in
qemuShimEventLoop(). However, it is still important to let the
event loop run after each API call (obviously).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>