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

What does this MR do and why?

This MR aims to clarify the https://docs.gitlab.com/ee/administration/lfs/#pure-ssh-transfer-protocol documentation by explain what happens when pure SSH protocol support is enabled.

Closes Improve Pure SSH protocol for LFS docs to make ... (#474567 - closed)

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