Formula-Cookbook: Stop recommending rm_f in postinstall

- After all the work that went into
  https://github.com/Homebrew/brew/pull/17705, we
  don't want the docs disagreeing with what CI says
  the style should be.

docs/.rubocop.yml

@@ -32,3 +32,7 @@ Style/RedundantRegexpArgument:
 # Want to be able to display partial formulae in the docs.
 Style/TopLevelMethodDefinition:
   Enabled: false
+
+# Formulae and Casks no longer use `rm_f`/`rm_rf`, so the docs need to match.
+Lint/NonAtomicFileOperation:
+  Enabled: false

docs/Formula-Cookbook.md

@@ -995,7 +995,7 @@ class Foo < Formula
   url "https://example.com/foo-1.0.tar.gz"
 
   def post_install
-    rm_f pkgetc/"cert.pem"
+    rm pkgetc/"cert.pem" if File.exist?(pkgetc/"cert.pem")
     pkgetc.install_symlink Formula["ca-certificates"].pkgetc/"cert.pem"
   end
   # ...