update SkMallocPixelRef docs

 - no more color tables
 - explicitly mention that MakeAllocate does not initialize the pixels

Change-Id: I98b9ef1fecc36ea5d9acbae66ccd8979810c2760
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206265
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed