RedisModule_HashSet call must end with NULL

Extended the RedisModule_HashSet doc to mark that each call must end with NULL

Author: gkorland

src/module.c

@@ -2150,7 +2150,9 @@ int RM_ZsetRangePrev(RedisModuleKey *key) {
  *
  * The function is variadic and the user must specify pairs of field
  * names and values, both as RedisModuleString pointers (unless the
- * CFIELD option is set, see later).
+ * CFIELD option is set, see later). At the end of the field/value-ptr pairs, 
+ * NULL must be specified as last argument to signal the end of the arguments 
+ * in the variadic function.
  *
  * Example to set the hash argv[1] to the value argv[2]:
  *