clock_gettime.2: Add cross references and fix linter warnings

MFC after:	3 days

lib/libc/sys/clock_gettime.2

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 1, 2021
+.Dd July 8, 2022
 .Dt CLOCK_GETTIME 2
 .Os
 .Sh NAME
@@ -132,7 +132,9 @@ struct timespec {
 .Pp
 Only the super-user may set the time of day, using only
 .Fa CLOCK_REALTIME .
-If the system securelevel is greater than 1 (see
+If the system
+.Xr securelevel 7
+is greater than 1 (see
 .Xr init 8 ) ,
 the time may only be advanced.
 This limitation is imposed to prevent a malicious super-user
@@ -185,7 +187,9 @@ The clock IDs
 .Fa CLOCK_UPTIME_FAST ,
 .Fa CLOCK_UPTIME_PRECISE ,
 .Fa CLOCK_SECOND
-are FreeBSD extensions to the POSIX interface.
+are
+.Fx
+extensions to the POSIX interface.
 .Sh HISTORY
 The
 .Fn clock_gettime ,