Minor developer documentation cleanups

[erooke]

While working on other things I stumbled on typos and minor mistakes in the developer facing documentation:

1. The graph data structure claims to only represent acyclic graphs but it has no problems with cycles.
2. The graph data structure claims that it only works with maps which aren't surjective to preserve the graph structure. What it actually requires is an injective map.
3. Minor typos here and there.