Improve Pure SSH protocol for LFS docs to make clarify SSH vs. HTTP connections

https://docs.gitlab.com/ee/administration/lfs/#pure-ssh-transfer-protocol describes how to enable Support the pure SSH-based LFS protocol for git... (&11872 - closed) but it doesn't include any detail on what enabling this feature means for HTTP based operations.

When gitlab_shell['lfs_pure_ssh_protocol'] = true is enabled, and the git-lfs client is v3.5.0+, it will always try the 'Pure SSH protocol' first and cannot be overridden on the command line AFAIK.