[Discovery] Add desc string for Mac permission for local network access

Translation check skipped because this new permission on mac isn't
available for testing yet. Will add translation screenshot later.

String is designed and approved by UX writer dwarren@. Design doc
linked in b/328310847#comment5

Bug: b/328310847
Change-Id: I0d161810a0cf2eda0fc0fbc517a027ec93a264bd
Skip-Translation-Screenshots-Check: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5615204


Commit-Queue: Muyao Xu <muyaoxu@google.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313613}