Explicitly mention that backticks should be used going forward

http://crrev.com/c/chromium/src/+/2253463/comment/2c9fbf8e_41d26e34/
mentions that we are less concerned about being consistent and care more
about doing the right thing going forward.
Turn this into an explicit rule here so people don’t have to go through
blame and dig up CLs where this is discussed.

Change-Id: I6e9344280c4ab336a717216f7b2f9c29de14261d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4649209
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Christoph Schlosser <cschlosser@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1174784}