correct comment about git submodules

BUILD

@@ -109,8 +109,11 @@ shell_command(
  tools=["cp"],
  # execution_dependencies allows pants to invalidate the output
  # of this command if the .gitmodules file changes (for example:
- # if a submodule gets updated to a different commit).
- # Theoretically, nothing else should modify .git/modules/.
+ # if a submodule gets updated to a different repo).
+ # Sadly this does not get invalidated if the submodule commit
+ # is updated. In our case, that should be rare. To work around
+ # If you update a submodule,
+ # this, kill the `pantsd` process after updating a submodule.
  execution_dependencies=[":gitmodules"],
  output_dependencies=[":gitmodules"],
  output_directories=[".git/modules"],