Document / elaborate on ssh host name configuration

[raymondbutcher]

The SSH config parser is building a fake hostname to get the user's preferred username or identityfile.

This, almost unintentionally, allows for user specific customisation like this:

Host *.ssha
    IdentityFile ~/.ssh/key1

Host *.myproject.ssha
    User someone
    IdentityFile ~/.ssh/key2

This should be considered further and then documented.