Expand info on login session persistence

Article: https://web.hypothes.is/help/how-long-do-i-remain-logged-in-to-hypothesis/

User feedback: Using latest Opera browser, Hypothes.is requires constant login for each new page and often it logs out on its own just after few moments(less than an hour)

Notes from support: users who block 3rd party cookies also cannot persist login across browser tabs.

We should spell this out more clearly.

Also, is it possible to be logged out in the middle of a session? If so, what happens? This is related to an accessibility roadmap item: https://github.com/hypothesis/product-backlog/issues/783

Review the linked login-session article and the support note about third-party cookies, then check the related accessibility roadmap issue 783 for context. Done means the article clearly explains persistence across pages and tabs, cookie restrictions, and what users should expect if they are logged out mid-session.

Written by the indexing model from the issue text.