Update comments for Timestamp JSON format.

Clarify that JSON parser are required to accept both UTC and other
timezone offsets.

Fixes issue 3093

src/google/protobuf/timestamp.proto

@@ -103,7 +103,9 @@ option objc_class_prefix = "GPB";
 // {hour}, {min}, and {sec} are zero-padded to two digits each. The fractional
 // seconds, which can go up to 9 digits (i.e. up to 1 nanosecond resolution),
 // are optional. The "Z" suffix indicates the timezone ("UTC"); the timezone
-// is required, though only UTC (as indicated by "Z") is presently supported.
+// is required. A proto3 JSON serializer should always use UTC (as indicated by
+// "Z") when printing the Timestamp type and a proto3 JSON parser should be
+// able to accept both UTC and other timezones (as indicated by an offset).
 //
 // For example, "2017-01-15T01:30:15.01Z" encodes 15.01 seconds past
 // 01:30 UTC on January 15, 2017.