Improve networking.md documentation per @jamtur01

Besides catching some typos and awkward sentences, @jamtur01 had several
more thorough changes to suggest:

 *  He illustrated the correct way to delimit "Note" paragraphs in
     Docker documentation.

 *  He helped point out where I was presuming an Ubuntu host running
    Docker, so that I could re-word those sections to be specific that
    the advice only applied to Ubuntu (these mostly related to how to
    set server-wide options and restart the server).

 *  He is happy to have "Ethernet" capitalized even where Linux
    documentation would render it with an ugly lower-case first letter.

Docker-DCO-1.1-Signed-off-by: Brandon Rhodes <brandon@rhodesmill.org> (github: brandon_rhodes)