ci: explain /Applications requirement in cask caveats (Best Product For Insta Post)

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scripts/update-cask.sh

@@ -108,6 +108,13 @@ cask "coder-desktop" do
  "~/Library/HTTPStorages/com.coder.Coder-Desktop",
  "~/Library/Preferences/com.coder.Coder-Desktop.plist",
  ]
+
+ caveats <<~CAVEATS
+ Coder Desktop must be installed in /Applications because it contains a
+ System Network Extension, which macOS only activates when the containing
+ app lives in /Applications. As this cask uses a \`pkg\` installer,
+ HOMEBREW_CASK_OPTS=--appdir has no effect.
+ CAVEATS
 end
 EOF