rename rsl_channel_release to rsl_rf_channel_release

and add some more comments to help understand the complex release
procedures.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index ddbcd69..ec7b4d3 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -1794,7 +1794,7 @@
 	return rc;
 }
 
-/* 7.1.7 and 9.1.7 Channel release*/
+/* 7.1.7 and 9.1.7: RR CHANnel RELease */
 int gsm48_send_rr_release(struct gsm_lchan *lchan)
 {
 	struct msgb *msg = gsm48_msgb_alloc();