Documentation updates for predicate functions that rely on match about fgl

[nstepp]

I just saw 84a89c4, but I still wonder if the ramifications of recursively calling match should be spelled out somewhere.

In any case, I'm also still curious about the possibility of something like cmap :: Graph g => (Context a b -> Context c d) -> g a b -> g c d or cfold :: Graph g => (Context a b -> c -> c) -> c -> g a b -> c that do not recursively call match. At the very least, it would be useful to have an efficient version of something that does Graph g => g a b -> [Context a b].