Add developer documentation on how to debug missing auto-release pools

Change-Id: Ie3942210ab5bafea22d65d6f7c9a099e40ee6b73
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/corelib/kernel/qcore_mac_objc.mm

@@ -129,6 +129,16 @@ QT_USE_NAMESPACE
 QT_NAMESPACE_ALIAS_OBJC_CLASS(QMacAutoReleasePoolTracker);
 QT_BEGIN_NAMESPACE
 
+
+/*
+    Manages a scoped auto-release pool.
+
+    To track autoreleases without any pools in place, such as in main()
+    before the runloop has started, export OBJC_DEBUG_MISSING_POOLS=YES
+    and break in objc_autoreleaseNoPool, e.g.:
+
+        br set -n objc_autoreleaseNoPool -c "[((NSObject*)$r14) class] == [QNSWindow class]"
+*/
 QMacAutoReleasePool::QMacAutoReleasePool()
     : pool([[NSAutoreleasePool alloc] init])
 {