CURLOPT_HEADERFUNCTION.3: Document that size is always 1

For compatibility with `fwrite`, the `CURLOPT_HEADERFUNCTION` callback
is passed two `size_t` parameters which, when multiplied, designate the
number of bytes of data passed in. In practice, CURL always sets the
first parameter (`size`) to 1.

This practice is also enshrined in documentation and cannot be changed
in future. The documentation states that the default callback is
`fwrite`, which means `fwrite` must be a suitable function for this
purpose. However, the documentation also states that the callback must
return the number of *bytes* it successfully handled, whereas ISO C
`fwrite` returns the number of items (each of size `size`) which it
wrote. The only way these numbers can be equal is if `size` is 1.

Since `size` is 1 and can never be changed in future anyway, document
that fact explicitly and let users rely on it.

Reported-by: Frank Gevaerts
Commit-message-by: Christopher Head

Ref: https://github.com/curl/curl/pull/2787

Fixes https://github.com/curl/curl/issues/4758

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