Add autougprade-mixed.md and remove fallback info

The console message informing users about the mixed content autoupgrade
experiment had a typo in it, this has been fixed in 74, but since the
experiment will be run in M73 while it's in beta, this CL adds a doc at
the URL with the typo pointing to the correct doc. Additionally, since
we are not running the experiment with fallback, that section was
removed from the doc.

Bug: 938916
Change-Id: If791704151b1ea541d66bb1a280e31c0967320c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534934
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643400}
2 files changed