Update ssh env vars description

[aryzing]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Adds a description of the GIT_SSH_COMMAND env variable.

Context

Seems when this section was first written, GIT_SSH_COMMAND did not yet exist or wasn't as popular as it seems to be now. Especially when handling multiple keys across different git hosting services, it's a convenient way to quickly select the a given key for a given service.