Added safety clarification to create_surface
This clarifies that the window that a surface is created on must be kept
alive for the lifetime of the surface. This requirement and a proposal
to change it are described in
@@ -1454,7 +1454,8 @@ impl Instance {
///
/// # Safety
///
- /// - Raw Window Handle must be a valid object to create a surface upon.
+ /// - Raw Window Handle must be a valid object to create a surface upon and
+ /// must remain valid for the lifetime of the returned surface.
pub unsafe fn create_surface<W: raw_window_handle::HasRawWindowHandle>(
&self,
window: &W,
