Update `art::down_cast`'s documentation about the use of `dynamic_cast`.

The implementation of `art::down_cast` has not used `dynamic_cast` in
debug mode since commit 07d579faa0abc835e8db970ff48b6f79cdc67f8f (see
https://android.googlesource.com/platform/art/+/07d579faa0abc835e8db970ff48b6f79cdc67f8f%5E%21/#F8).

Test: n/a (comment-only change)
Change-Id: I9f1b5b27406dcdab8220b0b1e84a5d4f7681f510
1 file changed