reflect: update Type.FieldByName documentation

Fixes #61495

Change-Id: I9abaf3613c797006b803dcb1dbee16f25ffb7516

Change-Id: I9abaf3613c797006b803dcb1dbee16f25ffb7516
GitHub-Last-Rev: 7bc2cba
GitHub-Pull-Request: #61645
Reviewed-on: https://go-review.googlesource.com/c/go/+/514035
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/reflect/type.go

@@ -172,6 +172,9 @@ type Type interface {
 
 	// FieldByName returns the struct field with the given name
 	// and a boolean indicating if the field was found.
+	// If the returned field is promoted from an embedded struct,
+	// then Offset in the returned StructField is the offset in
+	// the embedded struct.
 	FieldByName(name string) (StructField, bool)
 
 	// FieldByNameFunc returns the struct field with a name
@@ -186,6 +189,10 @@ type Type interface {
 	// and FieldByNameFunc returns no match.
 	// This behavior mirrors Go's handling of name lookup in
 	// structs containing embedded fields.
+	//
+	// If the returned field is promoted from an embedded struct,
+	// then Offset in the returned StructField is the offset in
+	// the embedded struct.
 	FieldByNameFunc(match func(string) bool) (StructField, bool)
 
 	// In returns the type of a function type's i'th input parameter.