[Documentation] Update documentation for Reference processing performance

Problem to solve

In !34741 (merged) we introduced optimization for Banzai::Filter::ReferenceFilter. As part of our rollout plan: #230689 (closed), we need to update documentation related to Reference processing: https://docs.gitlab.com/ee/development/reference_processing.html

Proposal

We should update #performance section. It should contain details about performance optimization introduced in !34741 (merged), about updating nodes processed nodes, instead of parsing the whole document again for each filter.