Doc update: Workaround for Gitaly cannot call itself under certain network configuration

Gitaly has certain RPCs in which a given Gitaly node might call back into itself. FetchSourceBranch is one of them.

Under certain network configuration, Gitaly may be unable to call itself. Once of those configuration is described here, where Gitaly is behind an AWS NLB where client IP Preservation is enabled.

The addresses Gitaly uses to call itself are those configured in Gitaly. This is problematic because, in the case of Sidekiq, the address to reach might need to be the DNS name of the NLB, but Gitaly cannot use that same address.

The workaound, as explained here, is to redirect the NLB address to the Gitaly node's own IP address in the /etc/hosts file.

This issue is to track to work to update the documentation with that workaround.