docs: clarify PAM config is a file entry; add uninstall instructions

Formula/ahfail.rb

@@ -32,12 +32,15 @@ class Ahfail < Formula
       "/etc/pam.d/screensaver"
     end
     <<~EOS
-      Add this line to #{pam_file} after the existing auth entries (requires sudo):
+      To activate, add this line to #{pam_file} after the existing auth entries
+      (requires sudo — this is a config file entry, not a shell command):
 
         auth optional #{lib}/ahfail/libahfail_pam.so
 
-      After upgrading ahfail, re-run `brew install ahfail` and update the path
-      in #{pam_file} if it changed.
+      To uninstall cleanly, remove that line before running `brew uninstall ahfail`:
+
+        sudo sed -i '' '/ahfail/d' #{pam_file}
+        brew uninstall ahfail
     EOS
   end