internal/godoc/internal/doc: mark comparable as predeclared

Fixes golang/go#51141.

Change-Id: Id92108ff3c561ab33a119c395cbad09555b864f5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/385198
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/internal/godoc/internal/doc/reader.go b/internal/godoc/internal/doc/reader.go
index a1f9141..16b4c41 100644
--- a/internal/godoc/internal/doc/reader.go
+++ b/internal/godoc/internal/doc/reader.go
@@ -928,6 +928,7 @@
 	"any":        true,
 	"bool":       true,
 	"byte":       true,
+	"comparable": true,
 	"complex64":  true,
 	"complex128": true,
 	"error":      true,