Trim: firefox launcher, debugger-arm migration, comments and docs

- firefox: the prefs.js patterns built once; the extension link condition
  without the redundant desktopFix (it implies a pref).
- install.sh steamvr-debugger-arm: no second copy of the old-marker
  migration (cleanup, run on every switch before the arm unit exists,
  already does it).
- steam-ui-patches: shorter state option description; lib's description
  mentions extraArgs.
- README: uiPatches.patches fields in the options table (moved from
  docs/ui-patches.md); docs/ui-patches.md: the finder helpers;
  docs/steamvr-debugger.md: repeated sentences removed.

modules/cleanup/check.nix

@@ -203,11 +203,9 @@ pkgs.runCommand "cleanup-check" { nativeBuildInputs = [ cleanup pkgs.jq ]; } ''
   arm() { bash ${../../install.sh} steamvr-debugger-arm; }
   fresh e
   printf '{\n   "steamvr" : {}\n}\n' > $V
-  touch $S/steamvr-debugger                          # old marker: migrated
   echo activating > $STUB/state
   res=$(arm); echo "$res"
   [ "$(cat $S/steamvr-debugger.armed)" = absent ] || fail "armed"
-  gone $S/steamvr-debugger
   [ "$(jq -c .VRWebHelper $V)" = '{"DebuggerEnabled":true}' ] || fail "arm: $(cat $V)"
   there $root/run/steam-frame-nix/steamvr-debugger-restore
   [ "$(grep -c daemon-reload $STUB/log)" = 1 ] || fail "arm reload"