Improve docs on Accept header and content negotiation.

The user guide did not make it clear that Grails ignores the Accept header for
content negotiation unless explicitly configured to do so. Hence why the issue
GRAILS-9810 was raised.

I have now explained how to activate this feature and also how best to use it,
i.e. don't use `withFormat()` for HTML pages.
Added warning to install-plugin about using BuildConfig instead
Merge branch '2.1.x' into 2.2.x