pretty: Note that Compare doesn't compare types.
diff --git a/pretty/public.go b/pretty/public.go
index 2ad2067..fbc5d7a 100644
--- a/pretty/public.go
+++ b/pretty/public.go
@@ -160,12 +160,27 @@
 // side it's from. Lines from the a side are marked with '-', lines from the
 // b side are marked with '+' and lines that are the same on both sides are
 // marked with ' '.
+//
+// The comparison is based on the intentionally-untyped output of Print, and as
+// such this comparison is pretty forviving.  In particular, if the types of or
+// types within in a and b are different but have the same representation,
+// Compare will not indicate any differences between them.
 func Compare(a, b interface{}) string {
 	return CompareConfig.Compare(a, b)
 }
 
 // Compare returns a string containing a line-by-line unified diff of the
 // values in got and want according to the cfg.
+//
+// Each line in the output is prefixed with '+', '-', or ' ' to indicate which
+// side it's from. Lines from the a side are marked with '-', lines from the
+// b side are marked with '+' and lines that are the same on both sides are
+// marked with ' '.
+//
+// The comparison is based on the intentionally-untyped output of Print, and as
+// such this comparison is pretty forviving.  In particular, if the types of or
+// types within in a and b are different but have the same representation,
+// Compare will not indicate any differences between them.
 func (cfg *Config) Compare(a, b interface{}) string {
 	diffCfg := *cfg
 	diffCfg.Diffable = true