[synchronous-mode] Document for configuration of synchronous mode#672

Merged

qiluo-msft merged 4 commits intosonic-net:masterfrom shi-su:sync_mode_cfg

Nov 4, 2020

Commits on Sep 4, 2020

Add a document for configuration of synchronous mode
shi-su
committed

Commits on Sep 5, 2020

Address review comments
shi-su
committed

Commits on Sep 9, 2020

Add discussion about applying the config in configDB and L2 switch mode
shi-su
committed
Add a missing section number
shi-su
committed