Docs: Add a warning about fields vs. source filtering.
Docs: Add a warning about fields vs. source filtering.

Close #14470

strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test