documentation: curl_formadd field and file names are now escaped

Prior to 7.56.0, fieldnames and filenames were set in Content-Disposition
header without special processing: this may lead to invalid RFC 822
quoted-strings.
7.56.0 introduces escaping of backslashes and double quotes in these names:
mention it in the documentation.

Reported-by: daboul on github
Closes #3361