doc: add comments about update_key() and get_syms() order

I remember we had a comment about this, but I can't find it. So add it
again.

Signed-off-by: Ran Benita <ran234@gmail.com>
interactive-x11: beef up select_events a bit

- Specify in detail which parts of the events we care about. In theory
  the X server should not bother us with things we didn't ask for. In
  practice it still does, but oh well.

- Use the _aux version of select_events. This is the correct one to use,
  the non-aux version is useless.

Signed-off-by: Ran Benita <ran234@gmail.com>