update docstring

Author: st--

src/TestUtils.jl

@@ -42,6 +42,10 @@ Test whether the prediction interface for `approx` works and whether in the
 conjugate case `posterior(approx, LatentGP(f, GaussianLikelihood(), jitter)(x), y)`
 gives approximately the same answer as exact GP regression.
 
+Should be satisfied by all approximate inference methods, but note that this
+does not currently apply for some approximations which rely on optimization
+such as `SparseVariationalApproximation`.
+
 Does not test `approx_lml`!
 
 Do not rely on this as the only test of a new approximation!