Cleanup terminology in docs

[DanielVandH]

I believe I've been a bit careless in some places with how I use the terms edges / segments and points / vertices. I should clean this up at some point, so that:

• edges are simply pairs (u, v) in the triangulation, but
• segments are the constrained edges.
  In this way, boundary edge might just refer to an edge that happens to be on the boundary, while a boundary segment should refer to a constrained edge appearing on the boundary. Similarly,
• points are the actual coordinates p[i] = (x[i], y[i]),
• while vertices are the references to the point, i.e. i
  Similarly, wherever I use BoundaryIndex it should probably just be GhostVertex or GhostIndex.

[DanielVandH]

Closing this as it is mentioned already now in #63 and is already in progress in the new-docs branch.