Add more detail on how DM checksums are done.
Add more detail on how DM checksums are done.

This will be linked to from Gold for image diffs that have
different digests but no pixel differences.

BUG=skia:
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1151153005

Review URL: https://codereview.chromium.org/1151153005

pcwalton/skia,vanish87/skia,rubenvb/skia,todotodoo/skia,vanish87/skia,ominux/skia,HalCanary/skia-hc,nvoron23/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,pcwalton/skia,noselhq/skia,tmpvar/skia.cc,HalCanary/skia-hc,nvoron23/skia,ominux/skia,todotodoo/skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,shahrzadmn/skia,google/skia,rubenvb/skia,HalCanary/skia-hc,google/skia,pcwalton/skia,Jichao/skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,vanish87/skia,tmpvar/skia.cc,todotodoo/skia,aosp-mirror/platform_external_skia,vanish87/skia,nvoron23/skia,ominux/skia,todotodoo/skia,pcwalton/skia,vanish87/skia,todotodoo/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Jichao/skia,vanish87/skia,Jichao/skia,todotodoo/skia,noselhq/skia,aosp-mirror/platform_external_skia,pcwalton/skia,Jichao/skia,ominux/skia,tmpvar/skia.cc,tmpvar/skia.cc,shahrzadmn/skia,nvoron23/skia,ominux/skia,nvoron23/skia,tmpvar/skia.cc,google/skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,ominux/skia,shahrzadmn/skia,vanish87/skia,noselhq/skia,qrealka/skia-hc,noselhq/skia,nvoron23/skia,google/skia,vanish87/skia,Jichao/skia,noselhq/skia,Jichao/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,pcwalton/skia,noselhq/skia,google/skia,rubenvb/skia,shahrzadmn/skia,pcwalton/skia,qrealka/skia-hc,HalCanary/skia-hc,shahrzadmn/skia,qrealka/skia-hc,todotodoo/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,ominux/skia,rubenvb/skia,google/skia,HalCanary/skia-hc,nvoron23/skia,qrealka/skia-hc,nvoron23/skia,noselhq/skia,rubenvb/skia,google/skia,ominux/skia,Jichao/skia,shahrzadmn/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,qrealka/skia-hc,noselhq/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,pcwalton/skia,HalCanary/skia-hc,todotodoo/skia,shahrzadmn/skia,Jichao/skia,ominux/skia,rubenvb/skia,shahrzadmn/skia,HalCanary/skia-hc,google/skia,Jichao/skia,shahrzadmn/skia,vanish87/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,todotodoo/skia,pcwalton/skia,qrealka/skia-hc,tmpvar/skia.cc,qrealka/skia-hc,qrealka/skia-hc,noselhq/skia,HalCanary/skia-hc,google/skia