[Reporting CSV Export] Add details to workarounds for "token expired" issue

[tsullivan]

A relatively common type of error seen for Reporting is: security_exception Root causes: security_exception: token expired.
These happen in deployments that use token-based authentication (SAML tokens) when creating the CSV report takes longer than time allows with the authentication that is cached in report job details.

The thing to note about this type of error is that the deployment is stable, it's just the size of a requested report is too large to complete within the time allowed by the authentication token available to the Reporting task.

Workarounds

• Users can work around this error by creating smaller reports: instead of one report covering a large time range, they could create multiple reports that cover segmented time ranges.
• Another workaround would be to increase xpack.security.authc.token.timeout, which is set to 20m by default.

Action item
This issue is to add the above workaround advice for workaround in the user-facing documentation here: https://www.elastic.co/guide/en/kibana/current/reporting-troubleshooting-csv.html#reporting-troubleshooting-csv-token-expired

Related issue: https://github.com/elastic/kibana-team/issues/1331

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)