Document the INFO+ log level query restriction

We only index on INFO and higher log level messages and sometimes the
only good thing to fingerprint in the logs is at DEBUG level, but that
won't work with elastic-recheck right now, so we have to document the
limitation.

Change-Id: I89fc35bca9f99ffdb1a0a71d88d041ac2215dd9d
Merge "Document the INFO+ log level query restriction"