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: I358e059604c470fdbbd0c05d662cf61bb56a0220

test/docker/run.sh

diff --git a/test/docker/run.sh b/test/docker/run.sh
index abf59b9..c5af6e1 100755
--- a/test/docker/run.sh
+++ b/test/docker/run.sh
@@ -35,7 +35,10 @@
 
     Options:
     --help|-h
-    --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.
     --depends-on-plugin-jar|-e      path to depends-on plugin JAR file
 
 EOF