Document how to use the FLAGS on DiffSync and DiffSyncModel

[dgarros]

Environment

• DiffSync version: 1.3.0

Proposed Functionality

Improve the documentation, specifically around the usage of the FLAGS for DiffSync class and DiffSyncModel class

For reference the flags are defined here > https://github.com/networktocode/diffsync/blob/master/diffsync/enum.py

[dgarros]

The doc has been updated