fix: add short hint in tools/modules/update on how force rebuild
I always forget exactly where I dropped the sentinel files.
# updated to the pinned versions without configuring or building them.
# If the argument `--build_only` is passed, the script will assume that the
# repositories are at the correct version and will configure and build them.
+#
+# To force a rebuild of llvm and/or clang:
+#
+# rm third_party/llvm/llvm/*.sentinel
+# rm third_party/llvm/llvm/tools/clang/*.sentinel
+# ./tools/modules/update.sh
# This section is used for cleanup during/after fetching.
TMPDIRS=()
