Add comment about maximum number of URC handlers

Author: sandeepmistry

src/Modem.h

@@ -76,7 +76,7 @@ class ModemClass {
   String _buffer;
   String* _responseDataStorage;
 
-  #define MAX_URC_HANDLERS 10
+  #define MAX_URC_HANDLERS 10 // 7 sockets + GPRS + GSMLocation + GSMVoiceCall
   static bool _debug;
   static ModemUrcHandler* _urcHandlers[MAX_URC_HANDLERS];
 };