docs: Explain Builder feature flags (#6581)

Each Builder feature flag now has a short comment explaining what
enabling it changes.

## Checklist
- [x] Verify latest-head CI.
- [x] Address both flag-description review comments and reply before
resolving their threads.
- [x] Document all five flags next to their declarations.
- [x] Verify the comments against the current flag call sites.
- [x] Review documentation impact: code comments only; no user-facing
docs change.
- [x] Run feature-flags typecheck, formatting, and diff checks.

Follow-up to #6576.

packages/feature-flags/src/flags.ts

@@ -1,6 +1,15 @@
-// Only for development, is not supposed to be enabled at all.
+// Development only: show internal components in the Builder Components panel.
+// Do not enable in general deployments.
 export const internalComponents = false;
+
+// Skip the unsupported-browser warning dialog in non-Chromium browsers.
 export const unsupportedBrowsers = false;
+
+// Show Resource settings beside URL props in the settings panel.
 export const resourceProp = false;
+
+// Use the collaboration relay when COLLAB_RELAY_URL is configured.
 export const multiplayer = false;
+
+// Show deprecated components in a separate Builder Components panel section.
 export const deprecatedComponents = false;