cmd/cue: document CUE_EXPERIMENT=embed

For now it suffices to reference the proposal placeholder, which will
soon be fully populated and link to the proposal that is being drafted
in https://cuelang.org/cl/1197180.

Fixes #3261.

Signed-off-by: Paul Jolly <[email protected]>
Change-Id: I14dc4de17985cd326a910b579e96a852f2ab9587
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197334
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: myitcv

cmd/cue/cmd/help.go

@@ -214,6 +214,9 @@ If an environment variable is unset or empty, sensible default setting is used.
 		evalv3
 			Enable the new CUE evaluator, addressing performance issues
 			and bringing a better disjunction algorithm.
+		embed
+			Enable support for embedded data files as described in
+			https://cuelang.org/discussion/3264.
 
 	CUE_DEBUG
 		Comma-separated list of debug flags to enable or disable, such as: