session-management.md: Proposed way of handling $XDG_RUNTIME_DIR

[bermudi]

Manually setting $XDG_RUNTIME_DIR is not trivial to do while staying compliant to the XDG spec as noted in issue #635. This proposed way handles $XDG_RUNTIME_DIR properly without having to install elogind and should suggested as its alternative

fixes #635

[ahesford]

I reject your assertion that managing the environment variable is cumbersome, and the alternative you propose is both known to have consistency issues and hasn't been touched in several years.

The lifetime requirement on XDG runtime directories is without merit, especially because the spec itself declares that applications must properly handle situations where the runtime directory contains stale content.