The ->display() function is not included on the pages about renderers, as it has nothing to do with renderers - it is part of the forms library, as documented here: http://docs.moodle.org/dev/Form_API
If you think you have identified a gap in the documentation, then this is a perfect opportunity to jump in and add the missing information yourself. Moodle docs is a wiki, so, once you have signed up for an account, anyone can contribute to it.
If you're not sure about any changes you are going to make to the docs, then I usually suggest to go ahead and make the changes and then post in the forums for someone to check them. The worst that can happen is that someone reverts your changes (but that is unlikely, unless you are posting obvious spam), usually they just get corrected (or left untouched, if there are no problems).