When the submitted changelist pane has focus, a CTRL+F lets you do an arbitrary text search, which includes changelist descriptions.
The only limitation is that it searches just those changelists that have been fetched from the server, so you may need to up the number retrieved. This is done via the “Number of changelists, jobs, branch mappings or labels to fetch at a time” setting which can be found by navigating to Edit->Preferences->Server Data.