Clarifying note about Security_Impact-None labels in docs

Bug: 
Change-Id: I9a6474effacd48e3f4e4230b61960b691335d99b
Reviewed-on: https://chromium-review.googlesource.com/792118
Commit-Queue: Andrew Whalley <awhalley@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519468}
1 file changed