network-health: update localization string descriptions

The first pass at these descriptions were very simple and caused
confusion during translation. Update the descriptions so they can be
more easily translated in the future.

Skipping the translation screenshots since these strings have already
been translated and this change simply updates the descriptions for the
future.

Bug: chromium:1098405
Skip-Translation-Screenshots-Check: True
Change-Id: I3e575b5ad93655f7a978f6f325cae0db7748c93f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288360
Commit-Queue: Trent Begin <tbegin@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788329}