doc/: switch from deprecated /var/run to /run
The FHS 3.0 [F] has deprecated /var/run in favor of /run. Current
distributions have done so for a long time (for example in Debian
since 6.0 “squeeze”), even though they provide a compatibility
symlink from /var/run to /run. But software like systemd have
started emitting warnings when using /var/run, for example for
its PIDFile directive, which pollutes the logs.