CURLOPT_DEBUGFUNCTION.3: warn about internal handles

CURLOPT_DEBUGFUNCTION.3: warn about internal handles

- Warn that the user's debug callback may be called with the handle
  parameter set to an internal handle.

Without this warning the user may assume that the only handles their
debug callback receives are the easy handles on which they set
CURLOPT_DEBUGFUNCTION.

This is a follow-up to f8cee8cc which changed DoH handles to inherit
the debug callback function set in the user's easy handle. As a result
those handles are now passed to the user's debug callback function.

Closes https://github.com/curl/curl/pull/12034