res_pjsip_outbound_registration: Extend documentation for "max_retries".

If the "max_retries" option is set to 0 then upon failure no
further attemps are made, so explicitly document the behavior.

ASTERISK-28602

Change-Id: I1e30daae9dd6c49ce18744164214d3def505acbf

res/res_pjsip_outbound_registration.c

@@ -81,6 +81,11 @@
 				</configOption>
 				<configOption name="max_retries" default="10">
 					<synopsis>Maximum number of registration attempts.</synopsis>
+					<description><para>
+						This sets the maximum number of registration attempts that are made before
+						stopping any further attempts. If set to 0 then upon failure no further attempts
+						are made.
+					</para></description>
 				</configOption>
 				<configOption name="outbound_auth" default="">
 					<synopsis>Authentication object(s) to be used for outbound registrations.</synopsis>