Formula-Cookbook: note when to remove revisions

Closes Homebrew/homebrew#46286.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

share/doc/homebrew/Formula-Cookbook.md

@@ -909,7 +909,7 @@ Homebrew provides two Formula methods for launchd plist files. `plist_name` will
 
 ## Updating formulae
 
-Eventually a new version of the software will be released. In this case you should update the `url` and `sha256`. Please leave the `bottle do ... end`  block as-is; our CI system will update it when we pull your change.
+Eventually a new version of the software will be released. In this case you should update the `url` and `sha256`. If a `revision` line exists, it should be removed. Please leave the `bottle do ... end`  block as-is; our CI system will update it when we pull your change.
 
 Check if the formula you are updating is a dependency for any other formulae by running `brew uses UPDATED_FORMULA`. If it is a dependency please `brew reinstall` all the dependencies after it is installed and verify they work correctly.