Re-add comment why connection pooling is turned off for MySQL

The added comment was added in
fb5548efcb but got lost during a
refactoring.

Change-Id: Iaaad17f7b9ff76555e7a2b5d4c02134d12a32013

gerrit-server/src/main/java/com/google/gerrit/server/schema/MySql.java

@@ -47,6 +47,10 @@ class MySql extends BaseDataSourceType {
 
   @Override
   public boolean usePool() {
+    // MySQL has given us trouble with the connection pool,
+    // sometimes the backend disconnects and the pool winds
+    // up with a stale connection. Fortunately opening up
+    // a new MySQL connection is usually very fast.
     return false;
   }
 }