update toolchains docs

third_party/toolchains/README.md

@@ -1,11 +1,30 @@
 # RBE toolchain configuration (Linux and Windows)
 
-- rbe_ubuntu2004: Autogenerated toolchain configuration for linux RBE.
-  Run `generate_linux_rbe_configs.sh` to regenerate.
+## Linux
+The `rbe_ubuntu2004` directory contains the autogenerated toolchain configuration for linux RBE.
+Run `generate_linux_rbe_configs.sh` to regenerate.
 
-- rbe_windows_bazel_5.2.0_vs2019: The autogenerated toolchain configuration for windows RBE.
-  Note that the bazel version denotes the bazel version that was used to generate the toolchain configuration. The configuration
-  can be use with different bazel versions (as long as it works).
-  See go/rbe-windows-user-guide
+## Windows
 
-- rbe_win2019: The windows dockerfile to rebuild the docker image we use on RBE workers. See go/rbe-windows-user-guide
+The `rbe_windows_bazel_6.3.2_vs2019` directory contains the autogenerated toolchain configuration for windows RBE.
+The configuration was generated by the `rbe_configs_gen` tool (just as the linux RBE config),
+but since it's a windows configuration, it needs to be run on a windows machine.
+
+In order to regenerate:
+- Clone `grpc` repository in a kokoro debug windows VM (see internal documentation for how to create one)
+- Run `generate_windows_rbe_configs.sh` to regenerate the configs locally on the VM.
+- Copy the generated configs back to your workstation (e.g. via `gcloud compute scp` command)
+
+Note that the bazel version denotes the bazel version that was used to generate the toolchain configuration. The configuration
+can be use with different bazel versions (as long as it works).
+
+Also see go/rbe-windows-user-guide
+
+## Windows RBE docker image
+
+The `rbe_win2019` directory contains the dockerfile to rebuild the docker image we use on RBE windows workers. See go/rbe-windows-user-guide.
+
+TODO(jtattermusch): Add instructions on how to rebuild the image on a kokoro debug windows VM.
+
+Note that for linux, we use a docker image under `tools/dockerfile` (since for Linux RBE docker image is simply of the testing docker images
+we maintain)