systemd: add Documentation=

Add the manual page and the homepage to the systemd unit. This is
exposed, for instance, via the `systemctl help irqbalance` command.

Signed-off-by: Lucas Werkmeister <mail@lucaswerkmeister.de>

misc/irqbalance.service

@@ -1,5 +1,7 @@
 [Unit]
 Description=irqbalance daemon
+Documentation=man:irqbalance(1)
+Documentation=https://github.com/Irqbalance/irqbalance
 ConditionVirtualization=!container
 
 [Service]