docs(process_registry): say in the argv helper why OOMPolicy is absent

tools/process_registry.py

@@ -126,7 +126,8 @@ def _worker_memory_max_bytes() -> int:
 
 def _systemd_scope_argv(binary: str, unit_name: str, *argv: str) -> List[str]:
     """``systemd-run --user --scope`` argv shared by the probe and real spawns.
-    ``--collect`` self-cleans the scope after exit; ``--unit`` names it for systemctl."""
+    ``--collect`` self-cleans the scope after exit; ``--unit`` names it for systemctl.
+    No ``OOMPolicy=``: transient scopes reject it on systemd <253 (#102486)."""
     return [
         binary, "--user", "--scope", "--quiet", "--unit", unit_name, "--collect",
         "--property", "MemoryAccounting=yes",