docs: give every command-line option a help text

scripts/export_native.py

@@ -574,9 +574,19 @@ def main() -> None:
     group = parser.add_mutually_exclusive_group(required=True)
     group.add_argument("--all", action="store_true", help="every platform")
     group.add_argument("--platform", type=platform_arg, help="a single platform")
-    parser.add_argument("--output-dir", default="dist/upstream")
-    parser.add_argument("--truth-dir", default="dist/truth")
-    parser.add_argument("--platforms-dir", default="platforms")
+    parser.add_argument(
+        "--output-dir", default="dist/upstream",
+        help="directory the corrected platform files go to (default: %(default)s)",
+    )
+    parser.add_argument(
+        "--truth-dir", default="dist/truth",
+        help="directory of the truth models generate_truth writes (default: "
+             "%(default)s)",
+    )
+    parser.add_argument(
+        "--platforms-dir", default="platforms",
+        help="platform YAML directory (default: %(default)s)",
+    )
     parser.add_argument(
         "--upstream-dir",
         default=DEFAULT_CACHE,