MT#55283 Add Description keys to systemd service files

Reported-by: lintian
Change-Id: I467b1021b6bc9df5b8b9b16b7a2c9d50881e50bd
(cherry picked from commit f8e4dd5a8a)
(cherry picked from commit 9bdca8aa5d)

debian/ngcp-rtpengine-daemon.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=NGCP RTP/media Proxy Daemon
+Documentation=man:rtpengine(8)
 After=network-online.target
 After=remote-fs.target
 Requires=network-online.target

debian/ngcp-rtpengine-recording-daemon.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=NGCP RTP/media Recording Daemon
+Documentation=man:rtpengine-recording(8)
 After=network-online.target
 After=remote-fs.target
 After=ngcp-rtpengine-recording-nfs-mount.service