ssh: document sub-shell mode

doc/README-SSH.md

@@ -4,6 +4,9 @@
 [![Demo](https://asciinema.org/a/22959.png)](https://asciinema.org/a/22959)
 
 ## Advanced usage (multiple SSH sessions from a sub-shell)
+This feature allows using regular SSH-related commands within a subprocess running user's shell.
+`SSH_AUTH_SOCK` environment variable is defined for the subprocess (pointing to the SSH agent, running as a parent process).
+This way the user can use SSH-related commands (e.g. `ssh`, `ssh-add`, `sshfs`, `git`, `hg`), while authenticating via the hardware device.
 [![Subshell](https://asciinema.org/a/33240.png)](https://asciinema.org/a/33240)
 
 ## Loading multiple SSH identities from configuration file

libagent/ssh/__init__.py

@@ -81,7 +81,8 @@ def create_agent_parser(device_type):
 
     g = p.add_mutually_exclusive_group()
     g.add_argument('-s', '--shell', default=False, action='store_true',
-                   help='run ${SHELL} as subprocess under SSH agent')
+                   help=('run ${SHELL} as subprocess under SSH agent, allowing '
+                         'regular SSH-based tools to be used in the shell'))
     g.add_argument('-c', '--connect', default=False, action='store_true',
                    help='connect to specified host via SSH')
     g.add_argument('--mosh', default=False, action='store_true',