arm64: update the "minimum maximum" comment.

Noticed while looking at riscv64. Looks like a bug, but actually nothing
we can do about it now or for the foreseeable future.

Bug: https://github.com/google/android-riscv64/issues/45
Test: treehugger
Change-Id: I2be81b2fd7095df40958a1f641d7b89cf5a8e41d