Install guide networking concepts sentence edits

Reduced repetition of the term Networking, and adjusted sentence
structure to improve clarity - edits centred on sentences about the
external network in particular.

Change-Id: Id6e03196c4d5f00dd8023b0f8d6ac3219176ca96
backport: none
Closes-Bug: #1325798
Merge "Install guide networking concepts sentence edits"