encoding/json: mention SyntaxError in Unmarshal doc comment

Change-Id: I71c9d9ef9d21a7ae9466d8c7b283fdfbba01f5a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/390734
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/encoding/json/decode.go

@@ -75,6 +75,8 @@ import (
 // either be any string type, an integer, implement json.Unmarshaler, or
 // implement encoding.TextUnmarshaler.
 //
+// If the JSON-encoded data contain a syntax error, Unmarshal returns a SyntaxError.
+//
 // If a JSON value is not appropriate for a given target type,
 // or if a JSON number overflows the target type, Unmarshal
 // skips that field and completes the unmarshaling as best it can.

src/encoding/json/scanner.go

@@ -27,6 +27,7 @@ func Valid(data []byte) bool {
 
 // checkValid verifies that data is valid JSON-encoded data.
 // scan is passed in for use by checkValid to avoid an allocation.
+// checkValid returns nil or a SyntaxError.
 func checkValid(data []byte, scan *scanner) error {
 	scan.reset()
 	for _, c := range data {
@@ -42,6 +43,7 @@ func checkValid(data []byte, scan *scanner) error {
 }
 
 // A SyntaxError is a description of a JSON syntax error.
+// Unmarshal will return a SyntaxError if the JSON can't be parsed.
 type SyntaxError struct {
 	msg    string // description of error
 	Offset int64  // error occurred after reading Offset bytes