Rename install_hook to set_index_flag

Now that all data is indexed (otherwise AAE won't work) there is no longer a notion of a hook.  All data written to KV is passed to Yokozuna for minimal indexing.  But now there is a flag `yz_index_content` to indicate to Yokozuna that the content should also be indexed.

* Rename the code in the appropriate places: yz_kv and yz_wm_index

* Update documentation, explain that everything is indexed and what the index flag does.