isdn/gigaset: remove EXPERIMENTAL tag from GIGASET_CAPI

The CAPI variant of the Gigaset drivers can, in combination with
capidrv, now fully replace the legacy ISDN4Linux variant. All
reported problems have been fixed. So remove the EXPERIMENTAL tag
from the Kconfig option selecting it, and adapt the documentation
accordingly to encourage users to switch to it.

Impact: documentation/status update, no functional change
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>