[mojo][doc] Add embedder info to the services page

This change adds information on how to register embedder-specific
interfaces with BrowserInterfaceBroker, as well as a note about renderer
kills when a binder for the requested interface is not found.

Change-Id: I53651b969454313617bd51abe949ae9e5a471b84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107740
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751427}