docs: correct context limitations (#722)

Author: gdalle

DifferentiationInterface/docs/src/explanation/advanced.md

@@ -16,10 +16,7 @@ Every context argument must be wrapped in a subtype of [`Context`](@ref) and com
 Right now, there are two kinds of context: [`Constant`](@ref) and [`Cache`](@ref).
 
 !!! warning
-    This feature is still experimental and will not be supported by all backends.
-    At the moment:
-    - `Constant` is supported by all backends except symbolic ones
-    - `Cache` is only supported by finite difference backends and [`AutoForwardDiff`](@ref), but it is not yet optimized
+    Not every backend supports every type of context. See the documentation on [Backends](@ref) for more details.
 
 Semantically, both of these calls compute the partial gradient of `f(x, c)` with respect to `x`, but they consider `c` differently: