Improve NoC arch tag documentation, and add ability to use equations in NoC tags

[vaughnbetz]

1. We are missing description of the mesh keyword; it should be added.
2. The documentation says positionx, positiony etc. can be approximate, but don't say why / what this means (we should say we use the closest router in the grid, so some imprecision is OK).
3. We should support expressions (same as the grid) in the NoC positionx, positiony and mesh parameters. This would give us a limited ability to model floating size FPGAs with NoCs. The documentation should mention this is doable, and give an example (probably we'd give one with a mesh).