Improve documentation around -H option:

 - unix://path/to/socket should read unix:///path/to/socket like the rest of the documentation (a slash was missing)
 - Explicitly mention that the -H option itself may be specified multiple times, rather than the value being specified multiple times for a single -H option.

Docker-DCO-1.1-Signed-off-by: Mike MacCana <mike.maccana@gmail.com> (github: mikemaccana)