docs: explain inconsistency of _{read,write}_all()

These functions are inconsistent with our normal conventions in that
they set an output variable to a specified value, even in the case that
an error is thrown.

Document very clearly that this should be considered exceptional.

https://bugzilla.gnome.org/show_bug.cgi?id=737451

gio/goutputstream.c

@@ -246,8 +246,15 @@ g_output_stream_write (GOutputStream  *stream,
  * is set to @count.
  * 
  * If there is an error during the operation %FALSE is returned and @error
- * is set to indicate the error status, @bytes_written is updated to contain
- * the number of bytes written into the stream before the error occurred.
+ * is set to indicate the error status.
+ *
+ * As a special exception to the normal conventions for functions that
+ * use #GError, if this function returns %FALSE (and sets @error) then
+ * @bytes_written will be set to the number of bytes that were
+ * successfully written before the error was encountered.  This
+ * functionality is only available from C.  If you need it from another
+ * language then you must write your own loop around
+ * g_output_stream_write().
  *
  * Returns: %TRUE on success, %FALSE if there was an error
  **/