message: Add a comment on the string substitution formula
diff: # Note that you MUST use "source" in order to run the script in the same process as the calling
  # script, allowing fill_packaging_opts.sh to fill variables for the caller.
  +# Input: "/absolute/path/src/server_manager/electron_app/something_action.sh"
  +# Output: "yarn do server_manager/electron_app/something"
  readonly ELECTRON_PATH='server_manager/electron_app/'
  readonly RELATIVE="${1#*/src/${ELECTRON_PATH}}"
  readonly YARN_COMMAND="yarn do ${ELECTRON_PATH}${RELATIVE%_action.sh}"
  