[css-navigation-1] Add another more obscure ISSUE.

This one mentions problems when starting a same-document navigation in
the middle of a cross-document navigation.

(This is part of importing the history from WICG/declarative-partial-updates.)

Author: dbaron

css-navigation-1/Overview.bs

@@ -156,6 +156,10 @@ as follows:
 	during the loading of the new document in a cross-document navigation,
 	since I think they reference concepts that are only present on the old document.
 
+	ISSUE: The above definitions of from and to apparently don't work right
+	if you start a same-document navigation (e.g., with {{History/pushState}})
+	in the middle of a cross-document navigation.
+
 	ISSUE: Once routemaps are defined more formally,
 	this should be defined in terms of the routemap definition
 	instead of referring directly to URLPattern.