docs: comment about CURLE_READ_ERROR returned by curl_mime_filedata

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

.SH AVAILABILITY

As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.

.SH RETURN VALUE

CURLE_OK or a CURL error code upon failure.

CURLE_OK or a CURL error code upon failure. CURLE_READ_ERROR is only an

indication that the file is not yet readable: it can be safely ignored at

this time, but the file must be made readable before the pertaining

easy handle is performed.

.SH EXAMPLE

.nf

 curl_mime *mime;