Update documentation to reflect new roles

The [architecture](https://github.com/apple/foundationdb/blob/master/documentation/sphinx/source/kv-architecture.rst) page is a bit outdated.

- Missing new roles
- I think Log router is the only new role that's not there
- Still assumes data distributor and rate keeper are part of the master

Open documentation/sphinx/source/kv-architecture.rst and compare its listed roles and master responsibilities with the current architecture. Update the page to include the Log router and stop describing the data distributor and rate keeper as part of the master; no test is specified, so completion is verified by reviewing the rendered documentation.

Written by the indexing model from the issue text.