Update Timestamp proto comment on the nanos field.

Detailed description...
Update Timestamp proto comment on the nanos field to clarify its relationship to the seconds field.

PiperOrigin-RevId: 808581123
diff --git a/src/google/protobuf/timestamp.proto b/src/google/protobuf/timestamp.proto
index fd0bc07..fd308bd 100644
--- a/src/google/protobuf/timestamp.proto
+++ b/src/google/protobuf/timestamp.proto
@@ -131,14 +131,15 @@
 // ) to obtain a formatter capable of generating timestamps in this format.
 //
 message Timestamp {
-  // Represents seconds of UTC time since Unix epoch
-  // 1970-01-01T00:00:00Z. Must be from 0001-01-01T00:00:00Z to
-  // 9999-12-31T23:59:59Z inclusive.
+  // Represents seconds of UTC time since Unix epoch 1970-01-01T00:00:00Z. Must
+  // be between -315576000000 and 315576000000 inclusive (which corresponds to
+  // 0001-01-01T00:00:00Z to 9999-12-31T23:59:59Z).
   int64 seconds = 1;
 
-  // Non-negative fractions of a second at nanosecond resolution. Negative
-  // second values with fractions must still have non-negative nanos values
-  // that count forward in time. Must be from 0 to 999,999,999
+  // Non-negative fractions of a second at nanosecond resolution. This field is
+  // the nanosecond portion of the duration, not an alternative to seconds.
+  // Negative second values with fractions must still have non-negative nanos
+  // values that count forward in time. Must be between 0 and 999,999,999
   // inclusive.
   int32 nanos = 2;
 }