This documents that cogl_texture_get_rowstride() is deprecated (or
rather it was a mistake that the api was ever published) and also
clarifies the rowstride argument documentation for
cogl_texture_get_data() to explain how it's automatically calculated
when 0 is passed to help avoid misleading people into thinking that
cogl_texture_get_rowstride() is an appropriate way to get a valid
rowstride for that.
Reviewed-by: Neil Roberts <neil@linux.intel.com>