ae1ea2aa moved trace-related functions from runtime/pprof to
runtime/trace, but missed a doc comment and a code comment. Update
these to reflect the move.

Change-Id: I6e1e8861e5ede465c08a2e3f80b976145a8b32d8
Reviewed-on: https://go-review.googlesource.com/12525Reviewed-by: Dmitry Vyukov <dvyukov@google.com>