fix last commit, and add documentation.

(We need some CI to catch at least the trivial mistakes like the one from the
last commit.)

install-mpsd-software-environment.py

@@ -300,8 +300,10 @@ def main():
             subp.add_argument("release", type=str, help="Release version to install or remove")
             if cmd in ["install", "reinstall"]:
                 subp.add_argument(
-                    "--toolchain",
-                    "--toolchains",
+                    "--toolchains", # first option defines attribute
+                                    # name `args.toolchains` in `args = parser_args()`
+                    "--toolchain",  # allow singular as alternative 
+                                    # (-> creates attribute `args.toolchains` if used)
                     type=str,
                     nargs="+",
                     default="NONE",