expand on sk_double_to_float() comment

I want to make sure we've got our reasoning straight
as we start to use this more.

Change-Id: I64b8351549a59e118f564e8505d42a0a620449c1
Reviewed-on: https://skia-review.googlesource.com/47840
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
1 file changed