docs(config): Clarify SSH client config limitations

[rpatterson]

These changes represent my attempts to understand how using SSHFS-Win works from the perspective of someone more familiar with Linux OpenSSH administration than Windows network share administration.

I didn't read the source before making these changes. I've confirmed most, maybe all, of the positive assertions in these changes from testing. There may be more clarifications that should be included that I didn't test because I don't need those use cases. Finally, my testing was hardly rigorous or exhaustive, so consider this a draft of how the documentation might be more helpful for users like myself. Review carefully, and I'll incorporate feedback to make these change more correct and complete.

Specifically, this change only documents the specific bits of SSH client configuration supported by SSHFS-Win that I needed. So there may be other caveats, use cases, etc. that should be clarified in these changes. Similarly, once I encountered SSH client configuration that must be specific to SSHFS-Win, I stopped testing which such configuration works both in the user's C:\Users\foo\.ssh\config configuration and just recommended keeping all configuration specific to SSHFS-Win in C:\Program Files\SSHFS-Win\etc\ssh_config.

I drew from the following issues in working out my own configuration. Once finalized, my aim is that these changes will synthesize and resolve these issues:

Fixes #444 Fixes #457