Fix Activity#getReferrer Javadoc

The getReferrer Javadoc relating to onNewIntent behaviour is
inconsistent with the onNewIntent Javadoc. The latter states:

"Note that {@link #getIntent} still returns the original Intent. You can
use {@link #setIntent} to update it to this new Intent."

We verified the new Javadoc update with test apps A, B, and C. Both A
and B launched C, which was a singleInstance activity.

Bug: 315760568
Test: described above
Change-Id: I0c95af3ab4780e2f878f5ea1c54d5013f460f8ed