improve documentation of control channel options

• Improve the documentation of the control channel options in ssh_config(5):
  • Clarify what happens at ControlMaster=ask, when the connection trial of
    another ssh(1) is rejected.
  • Document that a non-backgrounded master control channel process will remain
    open until the last connection has been closed, even after its session has
    ended. And also that in the meantime new clients can start using the master
    control channel.
  • Document the default values of the ControlPath and ControlPersist
    directives.
  • Minor spelling mistakes and improvements.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>