docs: Fixed reference to "android:windowContentTransitions"

Per bug, we should not be telling people to use
android:windowContentTransitions to specify custom transitions;
instead, they should use android:windowActivityTransitions (which
is set to 'true' by default in the Material theme).

See first comment for doc stage location.

bug: 19440221
Change-Id: I35696c595a8424eab27d73b62835ddd79e3ca109