Mark cross_group as experimental

While all of Serai can be argued as experimental, the DLEq proof is 
especially so, as it's lacking any formal proofs over its theory.

Also adds doc(hidden) to the generic DLEqProof, now prefixed with __.

crypto/dleq/Cargo.toml

@@ -31,7 +31,7 @@ transcript = { package = "flexible-transcript", path = "../transcript", features
 
 [features]
 serialize = []
-cross_group = ["multiexp"]
+experimental_cross_group = ["multiexp"]
 secure_capacity_difference = []
 
 # Only applies to cross_group, yet is default to ensure security