Changed add to group command to include -a (append) flag, for safety. In response to customer email.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #6741 from fredlf/suse-install-fix

Changed add to group command to include -a (append) flag, for safety. In...
Fixed numerous inconsistencies in command help text

* Replaced docker with Docker.
* Consistently used STDIN, STDOUT, STDERR.
* Consistently used TTY.
* Fixed some grammar and spelling issues.
* Fixed references to the index.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)