Documentation: --allow-master-master flag

This is a placeholder for me to update the docs to clarify `--allow-master-master` usage and clean up anywhere it may be unnecessary.

e.g. it's in the example for "a. Connect to replica, migrate on master" in the cheatsheet (https://github.com/github/gh-ost/blob/master/doc/cheatsheet.md) but master-master is not a default setup for a replication topology and probably shouldn't be in that example.

Also, is it still needed for "b. Connect to master"?

Searching the docs, it's not really described anywhere except in reference to other flags. Is search broken or do we not have a list of all possible flags somewhere with descriptions for each?