chan_dahdi: Clarify scope of callgroup/pickupgroup.

chan_dahdi: Clarify scope of callgroup/pickupgroup.

Internally, chan_dahdi only applies callgroup and
pickupgroup to FXO signalled channels, but this is
not documented anywhere. This is now documented in
the sample config, and a warning is emitted if a
user tries configuring these settings for channel
types that do not support these settings, since they
will not have any effect.

Resolves: #294