add notes

Author: Plasmatium

src/time/time.go

@@ -55,7 +55,9 @@
 // On such a system, t.Sub(u) may not accurately reflect the actual
 // time that passed between t and u. The same applies to other functions and
 // methods that subtract times, such as [Since], [Until], [Before], [After],
-// [Add], [Sub], [Equal] and [Compare].
+// [Add], [Sub], [Equal] and [Compare]. In some cases, you may need to strip
+// the monotonic clock or use an external source of time outside the current
+// process to get more accurate results.
 //
 // Because the monotonic clock reading has no meaning outside
 // the current process, the serialized forms generated by t.GobEncode,