Review audit events streaming documentation and tidy up after the custom headers feature is being released

The documentation for audit events streaming is a mix of API and UI guidance split by functionality. It is quite repetitive and might also not be easy to follow.

We should review the documentation before the final release of the custom headers feature and make sure that the documentation is tidy, makes sense, and follows all our styleguide rules. For instance:

One non-blocking thing is our use of the SVGs and having them conform more to: https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#gitlab-svg-icons. So rather than Select **{pencil}** at, we refer to the tooltip for the icon, with **{pencil}* in parenthesis. Let's do that as we tidy stuff up after this.