adjtime(2): document that delta can be NULL

The current verbiage somewhat indicates that always adjusts the time,
which hasn't been true as far back as I had the energy to `git blame`.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D57676

lib/libsys/adjtime.2

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 13, 2021
+.Dd June 19, 2026
 .Dt ADJTIME 2
 .Os
 .Sh NAME
@@ -46,6 +46,7 @@ makes small adjustments to the system time, as returned by
 advancing or retarding it
 by the time specified by the timeval
 .Fa delta .
+.Pp
 If
 .Fa delta
 is negative, the clock is
@@ -55,6 +56,13 @@ If
 .Fa delta
 is positive, a larger increment than normal
 is used.
+.Fa delta
+may also be a
+.Dv NULL
+pointer to fetch
+.Fa olddelta
+without making any adjustment.
+.Pp
 The skew used to perform the correction is generally a fraction of one percent.
 Thus, the time is always
 a monotonically increasing function.
@@ -65,7 +73,9 @@ may not be finished when
 is called again.
 If
 .Fa olddelta
-is not a null pointer,
+is not a
+.Dv NULL
+pointer,
 the structure pointed to will contain, upon return, the
 number of microseconds still to be corrected
 from the earlier call.