manual: Add a comparative example of 'clock_nanosleep' use

Add an illustrative example of how to express 'nanosleep' in terms of
'clock_nanosleep'.

manual/time.texi

@@ -3241,6 +3241,19 @@ different from @code{nanosleep}, which returns @math{-1} upon failure and
 sets the global variable @code{errno} according to the error encountered
 instead.
 
+Except for the return value convention and the way to communicate an error
+condition the call:
+
+@smallexample
+nanosleep (@var{requested_time}, @var{remaining_time})
+@end smallexample
+
+is analogous to:
+
+@smallexample
+clock_nanosleep (CLOCK_REALTIME, 0, @var{requested_time}, @var{remaining_time})
+@end smallexample
+
 The @var{clock} argument specifies the clock to use.
 @xref{Getting the Time}, for the @code{clockid_t} type and possible values
 of @var{clock}.  Not all clocks listed are supported for use with