Changes

vty: update help string for nokia_site model

Original Nokia BTS support in OsmoBSC was limited to MetroSite,
UltraSite and InSite, aka the Site family.  After the recent series
of fixes, the same code also supports Flexi Multiradio (actively
used in the present) and will most likely also work on Flexi EDGE
(intermediate between UltraSite and Flexi MR), although that model
is currently untested.

BTS model name "nokia_site" is thus outdated and inaccurate now.
It has been decided (see https://gerrit.osmocom.org/c/osmo-bsc/+/42729)
to keep this model name unchanged for now, given the uncertainty
of how Nokia BTS support will evolve in the future if and when
we implement support for IP Abis on Flexi family, potentially
followed by future support for subsequent IP-only models.

In the meantime, change vty help string to reflect current support
for both Site and Flexi families with the same model selection.

Change-Id: I8a2d91aacc984e6a818f9782e728674306f4ebe4