Docs: Add example of resetting index setting (#29048)
Docs: Add example of resetting index setting (#29048)

This commit adds an example using `null` to reset an index settings.

closes #22870
vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra