doc/rl2511: update allowVariants entry

## Highlights {#sec-nixpkgs-release-25.11-highlights}

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- Added `allowVariants` to gate availability of package sets like `pkgsLLVM`, `pkgsMusl`, `pkgsZig`, etc.

- Added `allowVariants` to gate availability of package sets like `pkgsLLVM`, `pkgsMusl`, `pkgsZig`, etc. This option will be removed in a future release.

## Backward Incompatibilities {#sec-nixpkgs-release-25.11-incompatibilities}