Give a command example with multiple languages.

tools/interop_matrix/README.md

@@ -8,8 +8,8 @@ from specific releases/tag, are used to test version compatiblity between gRPC r
 ## Step-by-step instructions for adding a GCR image for a new release for compatibility test
 We have continuous nightly test setup to test gRPC backward compatibility between old clients and latest server.  When a gRPC developer creates a new gRPC release, s/he is also responsible to add the just-released gRPC client to the nightly test.  The steps are:
 - Add (or update) an entry in `./client_matrix.py` file to reference the github tag for the release.
-- Build new client docker image(s).  For example, for java release `v1.9.9`, do
-  - `tools/interop_matrix/create_matrix_images.py --git_checkout --release=v1.9.9 --language=java`
+- Build new client docker image(s).  For example, for C and wrapper languages release `v1.9.9`, do
+  - `tools/interop_matrix/create_matrix_images.py --git_checkout --release=v1.9.9 --language cxx csharp python ruby php`
 - Verify that the new docker image was built successfully and uploaded to GCR.  For example,
   - `gcloud beta container images list --repository gcr.io/grpc-testing` shows image repos.
   - `gcloud beta container images list-tags gcr.io/grpc-testing/grpc_interop_java_oracle8` should show an image entry with tag `v1.9.9`.

tools/interop_matrix/create_matrix_images.py

@@ -81,8 +81,10 @@ argp.add_argument('--reuse_git_root',
                   default=False,
                   action='store_const',
                   const=True,                  
-                  help='reuse the repo dir. Reusing the repo can cause git '
-                  'checkout error if you switch between releases.')
+                  help='reuse the repo dir. If False, the existing git root '
+                  'directory will removed before a clean checkout, because '
+                  'reusing the repo can cause git checkout error if you switch '
+                  'between releases.')
 
 
 args = argp.parse_args()