Update comments to discuss leap smearing in detail.

Fixes #1890.

src/google/protobuf/timestamp.proto

@@ -40,17 +40,19 @@ option java_outer_classname = "TimestampProto";
 option java_multiple_files = true;
 option objc_class_prefix = "GPB";
 
-// A Timestamp represents a point in time independent of any time zone
-// or calendar, represented as seconds and fractions of seconds at
-// nanosecond resolution in UTC Epoch time. It is encoded using the
-// Proleptic Gregorian Calendar which extends the Gregorian calendar
-// backwards to year one. It is encoded assuming all minutes are 60
-// seconds long, i.e. leap seconds are "smeared" so that no leap second
-// table is needed for interpretation. Range is from
-// 0001-01-01T00:00:00Z to 9999-12-31T23:59:59.999999999Z.
-// By restricting to that range, we ensure that we can convert to
-// and from  RFC 3339 date strings.
-// See [https://www.ietf.org/rfc/rfc3339.txt](https://www.ietf.org/rfc/rfc3339.txt).
+// A Timestamp represents a point in time independent of any time zone or local
+// calendar, encoded as a count of seconds and fractions of seconds at
+// nanosecond resolution. The count is relative to an epoch at UTC midnight on
+// January 1, 1970, in the proleptic Gregorian calendar which extends the
+// Gregorian calendar backwards to year one.
+//
+// All minutes are 60 seconds long. Leap seconds are "smeared" so that no leap
+// second table is needed for interpretation, using a [24-hour linear
+// smear](https://developers.google.com/time/smear).
+//
+// The range is from 0001-01-01T00:00:00Z to 9999-12-31T23:59:59.999999999Z. By
+// restricting to that range, we ensure that we can convert to and from [RFC
+// 3339](https://www.ietf.org/rfc/rfc3339.txt) date strings.
 //
 // # Examples
 //