docs: comment about CURLE_READ_ERROR returned by curl_mime_filedata

docs/libcurl/curl_mime_filedata.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * 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
@@ -56,7 +56,10 @@ is retained.
 .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;