[Docs/Workbench] Improve SSH agent documentation
The current SSH-based documentation assumes that you have an SSH agent running with your keys in it (that's why they tell you to use
ssh -A). This isn't as common as we thought: people either aren't running the agent at all, or they don't add their keys before they connect. Spell out these requirements better, in both the User Guide and Workbench. The documentation should be consistent in both those places.