[infra] Add docs reminding users why we have our own deepequal.

Explored removing this in https://skia-review.googlesource.com/c/buildbot/+/372500

Change-Id: Ib361c11cf52d648d4eec0d482898df9604e47fbf
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/372509
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
diff --git a/go/deepequal/assertdeep/assertdeep.go b/go/deepequal/assertdeep/assertdeep.go
index 84b1250..50e5aa7 100644
--- a/go/deepequal/assertdeep/assertdeep.go
+++ b/go/deepequal/assertdeep/assertdeep.go
@@ -18,7 +18,11 @@
 // be enough for the average test.
 var superVerbose = false
-// Equal fails the test if the two objects do not pass reflect.DeepEqual.
+// Equal fails the test if the two objects do not pass a modified version of reflect.DeepEqual.
+// The modification (see infra/go/deepequal) will use the .Equal() method if defined on the type.
+// This is necessary for comparing structs that might differ on unexported fields, but be
+// practically the same. This concretely comes up often when deserialzing a time.Time, but applies
+// in a few other situations as well.
 func Equal(t sktest.TestingT, expected, actual interface{}) {
 	if !deepequal.DeepEqual(expected, actual) {
 		// The formatting is inspired by stretchr/testify's require.Equal() output.