docs: better explain multi-part byte range behavior

docs: better explain multi-part byte range behavior

- Better explain that if the requested range (--range or CURLOPT_RANGE)
  contains multiple ranges then the response contains meta information
  in addition to the requested bytes.

Prior to this change it was noted that a multiple part response was
returned as-is but not what that meant. In particular, meta information
is returned in addition to the requested bytes and that may have been
unexpected.

Reported-by: Ralf A. Timmermann
Fixes https://github.com/curl/curl/issues/16139
Closes https://github.com/curl/curl/pull/16150