update comments and rearrange SkVx.h

Just a little refactor no-op.

Change-Id: I1842a0190cd96c60da2fe3c7f88fa56c9f73af81
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/325681
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed