Update Branch list API documentation to reflect keyset pagination change

This is a follow up issue that came out of #208738 (comment 382705299). We need to update our documentation once we're ready to release the new Branch list keyset pagination and also add deprecating warning to the old pagination which is not performant if that's acceptable.

I think we can do this after #247047 (closed) and #247048 (closed) are done.

This is the issue that we change the default and/or deprecate old pagination. #247051 (closed)