commands: document the "event_log" class attribute

Add some notes explaining why it's instantiated at the Command class
level and not individual objects.

Change-Id: Ib8081bb8480e85f6d3dfc23953c6bbc6ecc64934
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309323
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>