feature gates: clarify beta enabling state

The feature gate usually gets changed to "enabled", but there are
exceptions. API groups are intentionally left as disabled.

Author: pohly

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -346,7 +346,7 @@ An *Alpha* feature means:
 
 A *Beta* feature means:
 
-* Enabled by default.
+* Usually enabled by default. Beta API groups are [disabled by default](https://github.com/kubernetes/enhancements/tree/master/keps/sig-architecture/3136-beta-apis-off-by-default).
 * The feature is well tested. Enabling the feature is considered safe.
 * Support for the overall feature will not be dropped, though details may change.
 * The schema and/or semantics of objects may change in incompatible ways in a