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}

styleguide/c++/c++-dos-and-donts.md

@@ -284,16 +284,17 @@ Good::Good() = default;
 
 ## Comment style
 
-References to code in comments should be wrapped in `` ` ` `` pairs. Codesearch uses
-this as a heuristic for finding C++ symbols in comments and generating
-cross-references for that symbol.
+References to code in comments should be wrapped in `` ` ` `` pairs. Codesearch
+uses this as a heuristic for finding C++ symbols in comments and generating
+cross-references for that symbol. Historically, Chrome also used `||` pairs to
+delimit variable names; codesearch understands both conventions and will
+generate a cross-reference either way. Going forward, prefer the new style even
+if existing code uses the old one.
 
 * Class and type names: `` `FooClass` ``.
 * Function names: `` `FooFunction()` ``. The trailing parens disambiguate
   against class names, and occasionally, English words.
-* Variable names: `` `foo_var` ``. Historically, Chrome also used `||` pairs to
-  delimit variable names; codesearch understands both conventions and will
-  generate a cross-reference either way.
+* Variable names: `` `foo_var` ``.
 * Tracking comments for future improvements: `// TODO(crbug.com/12345): ...`,
   or, less optimally, `// TODO(knowledgeable_username): ...`.  Tracking bugs
   provide space to give background context and current status; a username might