Changes

GSUP: PDP Context ID: start at 1

In OsmoHLR, PDP Contexts can have IDs <1-10>. 0 is used internally to
mark a profile slot as disabled:
https://gitea.osmocom.org/cellular-infrastructure/osmo-hlr/src/commit/d4b4548589f929d7c888ad213bd601ab2a421d8a/src/hlr_vty.c#L201

HLR Test TC_gsup_ul_subscriber_data in osmo-ttcn3-hacks also makes this
assumption:
https://gitea.osmocom.org/ttcn3/osmo-ttcn3-hacks/src/commit/85276b94c474bb649a0c4893b760c7b76f1f342e/hlr/HLR_Tests.ttcn#L1165-L1172

However the new GSUP implementation in PyHSS started its IDs at 0. I'll
send a patch to change it to start at 1 as well.

Update the documentation to reflect that 0 is not used as PDP Context ID.

Change-Id: I9c39002cb7f7dd81ae0f1f0f9d80df1aaf399f48