Docs: Reword note regarding _source for accuracy
Docs: Reword note regarding _source for accuracy

Previously it suggested _source was always present, when that is not the case.

Closes #8491

aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch