Note internal API default

- Keeps the `HOMEBREW_USE_INTERNAL_API` release reminder near
  its env config entry.
- Records why the next release should revisit the default.

Author: MikeMcQuaid

Library/Homebrew/env_config.rb

@@ -576,6 +576,7 @@ module EnvConfig
                      "if `--greedy` was passed when upgrading any cask on this list.",
       },
       HOMEBREW_USE_INTERNAL_API:                 {
+        # odeprecated: make default next release
         description: "If set, test the new beta internal API for fetching formula and cask data.",
         boolean:     true,
       },