Update comment on vulkan allocator.

Change-Id: I70991d5727172f72a61f073d0cd684da6ca43780
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209413
Commit-Queue: Greg Daniel <egdaniel@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Greg Daniel <egdaniel@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>