Documentation updates for predicate functions that rely on match

Functions that rely on `match` and also take a function involving `Context` show expected, but poorly documented behavior.

The issue comes up when user functions that are supposed to operate on `Context`s receive them as part of a recursive graph decomposition. As such, the graph that the contexts belong to gets smaller and smaller. User expectations are usually that if the starting graph has a particular context, that is the one that their function receives, as opposed to a context from a subsequent decomposition. This has come up before in #55 and #89.

I think this is at least a documentation fix, but it would also be useful to add some functions for mapping and folding over "full" contexts.

Would the maintainers be interested in pull requests for either?