Docs - Clarify Docker Runner Documentation

What does this MR do?

Clarifies the process of installing and registering a Docker Runner.

Why was this MR needed?

The current registration documentation doesn't explain that the given registration command will destroy the container. Additionally, I added a short(one-line) registration example to the installation page as well for the sake of completeness and step-by-step clarity. There was a bit of a disconnect between the implied context of the install doc and the register doc.

See the general guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html