Docs: add some note for access-log usage in extended mode with 'time-taken' identifier

When user sets up an access-log for EAP and configures it to use extended format with 'time-taken' identifier, this will print just empty string '-' character into the access-log. To configure it properly one has to also enable 'record-request-start-time' on appropriate listeners. We should add some note about this requirement into the Access log section in our configuration guide.

To try it, use steps described in this Jira.

Edited description because this JBEAP-5489 has been resolved.