osmo-bts-trx/loops.c: Add comments documenting TA + MS power control loop
osmo-bts-trx/loops.c: Add comments documenting TA + MS power control loop

The loops.c code is not very easily understood, so let's add some
comments to it.

As can be seen, there are functions of integer type which always return
0, and whose callers don't check for the return value.  This will be
adressed in subsequent patches.

Change-Id: Iafea07eb751ed85d29b214576bb0d31ea919cd72
Related: OS#1622, OS#1851

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts