doc: Add index entries for realtime and jackd in the manual.

* doc/guix.texi (Base Services): Add realtime and jackd indices to the
`pam-limits-service' service, as the examples shows how they can be
configured.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -10396,6 +10396,8 @@ to add @var{device} to the kernel's entropy pool.  The service will fail if
 @cindex session limits
 @cindex ulimit
 @cindex priority
+@cindex realtime
+@cindex jackd
 @deffn {Scheme Procedure} pam-limits-service [#:limits @code{'()}]
 Return a service that installs a configuration file for the