docs: list optional env vars commented out in compose files

CLAUDE.md

@@ -125,6 +125,11 @@ the `PASEO_*` daemon settings, `DOCKER_MODS` with the `INSTALL_PACKAGES` and
 of its most important member, even when a member on its own would rank lower.
 Within a group, the variable others configure comes first.
 
+Optional variables are listed but commented out, in the form
+`# - KEY=${KEY:-default}`, so the service runs without them and enabling one
+means uncommenting its line. The matching `.env.example` entry is commented out
+the same way (`# KEY=default`). Must-have and should-have variables stay active.
+
 Do not write the tier into the file as a comment — the order is the
 documentation. Where every variable is required, as in `alloy`, the tiers
 collapse and the existing grouping stands.