vcl: more VclPtr debugging docs in the README.lifecycle.

Change-Id: I49d06d54157e1e7c5b7ce2aa3f8917763de6826d

vcl/README.lifecycle

@@ -299,4 +299,28 @@ ways and often both.
 	cleanup methods, especially LoseFocus continue to work even
 	on disposed Window sub-class instances.
 
+** It crashes with some invalid memory ...
+
+        Assuming that the invalid memory is a Window sub-class itself,
+	then almost certainly there is some cockup in the
+	reference-counting; eg. if you hit an OutputDevice::release
+	assert on mnRefCount - then almost certainly you have a
+	Window that has already been destroyed. This can easily
+	happen via this sort of pattern:
+
+	ModelessDialog *pDlg = VclPtr<ModelessDialog>(nullptr /* parent */);
+	// by here the pDlg quite probably points to free'd memory
+	...
+
+	It is necessary in these cases to ensure that the *pDlg is
+	a VclPtr<ModelessDialog> instead.
+
+** It crashes with some invalid memory #2 ...
+
+	Often a ::dispose method will free some pImpl member, but
+	not NULL it; and (cf. above) we can now get various virtual
+	methods called post-dispose; so:
+
+	a) delete pImpl; pImpl = NULL; // in the destructor
+	b) if (pImpl && ...)           // in the subsequently called method