special help string for 'install'

Only 'install' needs to know about --ALL, etc.

mpsd-software-environment.py

@@ -383,6 +383,16 @@ def main():
                 "release", type=str, help="Release version to install or remove"
             )
             if cmd in ["install", "reinstall"]:
+                # "install" command needs additional documentation
+                if cmd == "install":
+                    help_part2 = (
+                        "Use '--toolchains ALL' to "
+                        "install all toolchains. If '--toolchain' is not "
+                        "specified, list available toolchains for the release "
+                        "(after environment has been prepared if not done yet)."
+                    )
+                else:
+                    help_part2 = ""
                 subp.add_argument(
                     "--toolchains",  # first option defines attribute
                     # name `args.toolchains` in `args = parser_args()`
@@ -392,12 +402,7 @@ def main():
                     dest="toolchains",
                     nargs="+",
                     default="NONE",
-                    help=(
-                        "List of toolchains to install (use '--toolchains ALL' to "
-                        "install all toolchains). If nothing is specified, list of"
-                        "available toolchains for the release would be shown after"
-                        "creating the environment."
-                    ),
+                    help=f"List of toolchains to {cmd}. " + help_part2,
                 )
                 subp.add_argument(
                     "--enable-build-cache",