Document that limiting scope of Elaticsearch to groups/projects disables global search

Problem statement

Currently in our documentation for Elasticsearch, we do not inform users whom are configuring the namespace limitations, that it will also disable use of ES in global search: https://docs.gitlab.com/ee/integration/elasticsearch.html#limiting-namespaces-and-projects

Proposal

We should document that this feature disables the use of ES when performing global searches, instead relying on PG. There will be a significant degradation of search results and quality when using PG, rather than what they probably expect (ES).