Documentation on knobs

We have knobs listed [here](https://github.com/apple/foundationdb/blob/master/fdbserver/Knobs.h) in the source code and some knobs have very good comments. And I could find the default values of the knobs [here](https://github.com/apple/foundationdb/blob/master/flow/Knobs.cpp). Considering how important knobs are to run the FoundationDB cluster, it could be useful to have them in the documentation. At least the ones that could effect the critical functionality.

Start with fdbserver/Knobs.h for the knob list and comments, then inspect flow/Knobs.cpp for default values. Identify the knobs affecting critical FoundationDB cluster functionality and determine where the project’s documentation should present them. Done means the selected knobs, descriptions, and defaults are documented.

Written by the indexing model from the issue text.