clarify what a "user" means

Created by: sqs

A prospective customer asked if our user licenses were concurrent:

Suppose I've logged into Sourcegraph on my desktop and at the same time I want to log in from my laptop. Do I need to log out from my desktop computer or I can use it at same time on both?

This docs update clarifies that the person does not need to log out. Users are counted as user accounts, not as sessions.