Add more detail on how DM checksums are done.

site/dev/testing/testing.md

 Correctness Testing
 ===================
 
 Skia correctness testing is primarily served by a tool named DM.
 This is a quickstart to building and running DM.
 
 ~~~
 $ ./gyp_skia
 $ ninja -C out/Debug dm
 $ out/Debug/dm -v -w dm_output
@@ skipping 109 matching lines @@
 dm_output/565/gm/bezier_quad_effects.png
                 ...
 ~~~
 
 The directories are nested first by sink type (`--config`), then by source type (`--src`).
 The image from the task we just looked at, "8888 image mandrill_132x132_12x12.astc-5-subsets",
 can be found at dm_output/8888/image/mandrill_132x132_12x12.astc-5-subsets.png.
 
 dm.json is used by our automated testing system, so you can ignore it if you
 like.  It contains a listing of each test run and a checksum of the image
-generated for that run.  (Boring technical detail: it is not a checksum of the
-.png file, but rather a checksum of the raw pixels used to create that .png.)
+generated for that run.
+
+### Detail <a name="digests"></a>
+Boring technical detail: The checksum is not a checksum of the
+.png file, but rather a checksum of the raw pixels used to create that .png.
+That means it is possible for two different configurations to produce
+the same exact .png, but have their checksums differ.
 
 Unit tests don't generally output anything but a status update when they pass.
 If a test fails, DM will print out its assertion failures, both at the time
 they happen and then again all together after everything is done running.
 These failures are also included in the dm.json file.
 
 DM has a simple facility to compare against the results of a previous run:
 ~~~
 $ ./gyp_skia
 $ ninja -C out/Debug dm
@@ skipping 30 matching lines @@
 That's the basics of DM.  DM supports many other modes and flags.  Here are a
 few examples you might find handy.
 ~~~
 $ out/Debug/dm --help        # Print all flags, their defaults, and a brief explanation of each.
 $ out/Debug/dm --src tests   # Run only unit tests.
 $ out/Debug/dm --nocpu       # Test only GPU-backed work.
 $ out/Debug/dm --nogpu       # Test only CPU-backed work.
 $ out/Debug/dm --match blur  # Run only work with "blur" in its name.
 $ out/Debug/dm --dryRun      # Don't really do anything, just print out what we'd do.
 ~~~