test/docker/run: Clarify --gerrit-war usage

Since the Dockerfile derives from an image that has already run init,
trying to use a gerrit WAR that is a substantially different version
will probably not work correctly. Add a note documenting that.

Change-Id: I1a3d815d0a19f2c045ba1426d503da5d92a63cef
diff --git a/test/docker/run.sh b/test/docker/run.sh
index 5b1894d..f323abd 100755
--- a/test/docker/run.sh
+++ b/test/docker/run.sh
@@ -35,7 +35,10 @@
-    --gerrit-war|-g             path to Gerrit WAR file
+    --gerrit-war|-g             path to Gerrit WAR file. Will likely not
+                                function correctly if it's a different
+                                MAJOR.MINOR version than the image version
+                                in test/docker/gerrit/Dockerfile.
     --events-plugin-jar|-e      path to events plugin JAR file