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

docs/libcurl/curl_formadd.3

@@ -175,7 +175,8 @@ Deprecated in 7.56.0. Before this release, field names were allowed to
 contain zero-valued bytes. The pseudo-filename "-" to read stdin is
 discouraged although still supported, but data is not read before being
 actually sent: the effective data size can then not be automatically
-determined, resulting in a chunked encoding transfer.
+determined, resulting in a chunked encoding transfer. Backslashes and
+double quotes in field and file names are now escaped before transmission.
 .SH RETURN VALUE
 0 means everything was ok, non-zero means an error occurred corresponding
 to a CURL_FORMADD_* constant defined in