Cluster: getKeysFromCommand() top comment improved.

src/db.c

@@ -951,8 +951,14 @@ int *getKeysUsingCommandTable(struct redisCommand *cmd,robj **argv, int argc, in
     return keys;
 }
 
-/* Return keys as an heap allocated array of integers. The length of the array
- * is returned by reference into *numkeys.
+/* Return all the arguments that are keys in the command passed via argc / argv.
+ *
+ * The command returns the positions of all the key arguments inside the array,
+ * so the actual return value is an heap allocated array of integers. The
+ * length of the array is returned by reference into *numkeys.
+ *
+ * 'cmd' must be point to the corresponding entry into the redisCommand
+ * table, according to the command name in argv[0].
  *
  * This function uses the command table if a command-specific helper function
  * is not required, otherwise it calls the command-specific function. */