Updates ML2 content in Networking intro

Rewrote introductory ML2 text for clarity. Made special mention of
the use case where ML2 will offer particular benefit. Updated the
note to emphasize that the deprecated plug-ins have been ported to
ML2.

Change-Id: Ie9273c73b265d6afe9a3603348c59806c1a9561c
Partial-Bug: #1217503
Merge "Updates ML2 content in Networking intro"