reflect: correct internal docs for uncommonType

This updates the doc to reflect the change in CL 19790 from 2016.

Change-Id: I1017babf6660aa3b4929755e2eccbe3168b7860c
Reviewed-on: https://go-review.googlesource.com/c/go/+/714880
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

Author: ianlancetaylor

src/reflect/type.go

@@ -314,7 +314,10 @@ const Ptr = Pointer
 
 // uncommonType is present only for defined types or types with methods
 // (if T is a defined type, the uncommonTypes for T and *T have methods).
-// Using a pointer to this struct reduces the overall size required
+// When present, the uncommonType struct immediately follows the
+// abi.Type struct in memory.
+// The abi.TFlagUncommon indicates the presence of uncommonType.
+// Using an optional struct reduces the overall size required
 // to describe a non-defined type with no methods.
 type uncommonType = abi.UncommonType