Fix TextView#setText misplaced documentation

In Ifaddb46d156c495a371789c6f32cfd67ffaaaef2, we improved the
documentation of TextView#setText to state an assumption that should
hold when subclasses override the method. However, the new phrase was
put in the wrong place. This CL fixes this.

Bug: none
Test: none
Change-Id: I918a6c533b9e4b34b3977a20e979a10db9741ebc