Add docs for feature reset API

[williamrandolph]

Here are some docs for the Feature Reset API. These should have been included in #69469, but were left for later.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[williamrandolph]

I have two questions about this work:

• How can I create a "warning" or "important" block to emphasize that this API shouldn't be used on production systems?
• Is there a way to loosen the check on the API response so that this test doesn't break when feature states added or removed.

[jaymode]

How can I create a "warning" or "important" block to emphasize that this API shouldn't be used on production systems?

For the warning, I am guessing something like the one on this page is good: https://github.com/elastic/elasticsearch/blob/master/docs/reference/index-modules.asciidoc

[jaymode]

Is there a way to loosen the check on the API response so that this test doesn't break when feature states added or removed.

Maybe the following from https://github.com/elastic/elasticsearch/tree/master/docs#readme will be helpful:

One interesting difference here is that you often want to match against the response from Elasticsearch. To do that you can reference the "body" of the response like this: // TESTRESPONSE[s/"took": 25/"took": $body.took/]

[williamrandolph]

I played with the substitutions, but I couldn't find a way to indicate a variable number of entries in a list. So we can either skip checking the result of this test, or we can require that if we add a feature state to the system, we have to add an entry here. Thoughts?

[debadair]

Left some suggestions--let me know if you have any questions!

[jaymode]

I think you should be able to do what we do for things like plugins:

// TESTRESPONSE[s/"plugins": \[[^\]]*\]/"plugins": $body.$_path/]

[jaymode]

Docs look good aside for Deb's suggestions.

[williamrandolph]

Backport commit: a46b8ea