Clarify, document Assignment related timers

Related: OS#3928
Change-Id: Iad31a5c6f83cd78793adf05a6af782ceacae8b11
diff --git a/src/bts.cpp b/src/bts.cpp
index 449e1ac..bcf4d1b 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -602,8 +602,10 @@
 
 	LOGP(DRLCMAC, LOGL_DEBUG, "Got IMM.ASS confirm for TLLI=%08x\n", tlli);
 
-	if (dl_tbf->m_wait_confirm)
+	if (dl_tbf->m_wait_confirm) {
+		/* Transition to FLOW in gprs_rlcmac_tbf::handle_timeout() when timer expires */
 		T_START(dl_tbf, T0, -2002, "assignment (AGCH)", true);
+	}
 
 	return 0;
 }