Correct errors in Event Feeds Processing HOWTO documentation

A review of the [Event Feeds Processing HOWTO](https://github.com/gchq/stroom-docs/tree/master/HOWTOs/EventFeeds) documentation identified a number of omissions and errors.

This issue is in place to mark their correction.

Open the HOWTOs/EventFeeds documentation linked in the issue and review its current content for the omissions and errors mentioned. Done means the identified documentation problems are corrected, but the issue does not specify which passages need changes or how to verify them.

Written by the indexing model from the issue text.