ktrace(1): add more xrefs

Following commit a87651e2ff add xrefs to intro(2) and sigaction(2),
and use a consistent form.

Suggested by:	kib, arrowd
Reviewed by:	kib (earlier)
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45555

(cherry picked from commit 0277c0c6f7)

usr.bin/ktrace/ktrace.1

@@ -48,9 +48,13 @@ The
 utility enables kernel trace logging for the specified processes.
 Kernel trace data is logged to the file
 .Pa ktrace.out .
-The kernel operations that are traced include system calls,
-.Xr namei 9
-translations, signal processing, and
+The kernel operations that are traced include system calls
+.Pq see Xr intro 2 ,
+file system path lookups
+.Pq Xr namei 9 ,
+signal processing
+.Pq Xr sigaction 2 ,
+and
 .Tn I/O .
 .Pp
 Once tracing is enabled on a process, trace data will be logged until
@@ -120,7 +124,9 @@ trace page faults
 trace
 .Tn I/O
 .It Cm n
-trace namei translations
+trace
+.Xr namei 9
+translations
 .It Cm p
 trace capability check failures
 .It Cm s
@@ -198,7 +204,9 @@ Disable tracing of all user-owned processes:
 .Xr dtrace 1 ,
 .Xr kdump 1 ,
 .Xr truss 1 ,
+.Xr intro 2 ,
 .Xr ktrace 2 ,
+.Xr sigaction 2 ,
 .Xr utrace 2 ,
 .Xr capsicum 4 ,
 .Xr namei 9