Document how to rebuild the performance counters registry cache

Occasionally an error is preventing AutoTx from starting up, showing a message like this in the log:

Cannot load Counter Name data because an invalid index '' was read from the registry.

Apparently this is due to the performance counters cache being corrupted, which has to be reset manually using the following command (administrator permissions required):

lodctr /r

Probably it would be a good idea to issue a hint in the log messages directly, so people know where to start looking.