[jiri] Inform users how to checkout JIRI_HEAD

If multiple repositories aren't on JIRI_HEAD it may be cumbersome
to manually checkout JIRI_HEAD in each one. Add documentation for
how this can be automated.

Change-Id: Ib7a14ac8ec5adf2a7fa41d25fd25fbc2e071a3f7
Reviewed-on: https://fuchsia-review.googlesource.com/c/jiri/+/455414
Reviewed-by: Haowei Wu <haowei@google.com>
Commit-Queue: Brian Bosak <bbosak@google.com>
diff --git a/project/project.go b/project/project.go
index c47edcb..8e82f97 100644
--- a/project/project.go
+++ b/project/project.go
@@ -2431,7 +2431,7 @@
 				msg = fmt.Sprintf("%s (%s)", msg, jirix.Color.Yellow("Not on JIRI_HEAD"))
 			}
 		}
-		jirix.Logger.Warningf("%s\n\n", msg)
+		jirix.Logger.Warningf("%s\n\nTo force an update to JIRI_HEAD, you may run 'jiri runp 'git checkout JIRI_HEAD'", msg)
 	}
 
 	if shouldFetchPkgs {