* Add info to docs about concurrent session limits

This parallels a [change in the Grafana Cloud docs](https://github.com/grafana/website/pull/3504) which was based on a kind and appreciated message from support. The limit does not apply to Grafana OSS, just to Enterprise and Cloud, as far as any of us know.

* Update docs/sources/enterprise/license-restrictions.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>