Clarify error page mechanism with regards to committed and async

The error page mechanism defined in section 10.9.2 can be invoked either by an explicit call to sendError or by an exception being thrown from a dispatch to filter(s) and servlet.

However, the current text is not clear what should be done in all possible states with regard to being committed and/or asynchronous states:
- exceptions thrown when the response is already committed
- exceptions thrown when the request is async started (or states beyond)
- sendError called by async thread
- sendError called during dispatch when async started

Start with section 10.9.2 of the Servlet specification and review how it describes error-page handling. Trace the four listed cases—committed responses, asynchronous requests, async-thread sendError, and sendError during dispatch—and clarify the expected behavior for each state. Done means the specification unambiguously covers all listed combinations.

Written by the indexing model from the issue text.