libssh2_channel_eof: clarify that it returns negative on errors

docs/libssh2_channel_eof.3

@@ -13,6 +13,7 @@ libssh2_channel_eof(LIBSSH2_CHANNEL *channel);
 
 Check if the remote host has sent an EOF status for the selected stream.
 .SH RETURN VALUE
-Returns 1 if the remote host has sent EOF, otherwise 0.
+Returns 1 if the remote host has sent EOF, otherwise 0. Negative on
+failure.
 .SH SEE ALSO
 .BR libssh2_channel_close(3)