Start moving docs to typeless APIs.

[jpountz]

A number of APIs (index creation, put mapping, index, etc.) will soon trigger
deprecation warnings unless users opt in for typeless APIs by passing
include_type_name=false.

Relates #15613

[elasticmachine]

Pinging @elastic/es-search-aggs

[jtibshirani]

The change makes sense to me, but I had a high-level question: we’ve discussed how it’s unfortunate that so many important APIs now require an extra parameter by default, and also that users will have to take action both in 7.0 to add the parameter, and in 8.0 to remove it. We’re brainstorming potential approaches to improve this set-up (such as adding a setting can be enabled instead of specifying a parameter in each request). Should we hold off on this change until we have a better handle on that, since it may affect whether we update all the REST documentation?

[jtibshirani]

You might be planning to do this later, but it would be nice to describe the include_type_name parameter in this high-level reference (as we do in get-mapping.asciidoc, etc.)

[jpountz]

Thanks for the feedback @jtibshirani. I raised a discussion at #15613 (comment).

[jtibshirani]

I'm going to close this because our plan has diverged a bit from when this was opened. We'll open new PRs to make the relevant documentation updates.