Incorrect description of composite indexes

The example [here](https://docs.hazelcast.org/docs/4.0.1/manual/html-single/index.html#indexing-queries) suggests that two indexes on two fields can help to execute `field1=A and field2=B`. For this, a composite index is needed.

It's correctly described here (as far as I can tell): https://docs.hazelcast.org/docs/4.0.1/manual/html-single/index.html#configuring-imap-indexes. It might be better to not attempt to explain index concepts here and refer to the other place. We actually do refer to it already.

In https://docs.hazelcast.org/docs/4.0.1/manual/html-single/index.html#configuring-imap-indexes I also miss the explanation that if two indexes can be used, which one is chosen.