docs: Noted how singleTop affects "up" navigation.

Docs are staged to:

http://asolovay.mtv:9411/training/implementing-navigation/ancestral.html#NavigateUp
http://asolovay.mtv:9411/guide/topics/manifest/activity-element.html#lmode

bug: 14527411
Change-Id: Ia414f2206339fc33739c8244f14140dd2e5d1607