sgsn_delete_pdp_ctx: Add documentation and assert assumptions

This function is only expected to be called if the GTP side of the PDP
ctx is still alive, since it will tear down the GTP side and then finish
the pending MS side if needed.

The asserts are added to ease debugging since it was noted that a few
callers were using this function without properly checking the status of
the pdp ctx.

Related: OS#4817
Change-Id: I4248e2e9846fec5ae2c8557384da2deb86668c50