[DOCS] Update docs for 'synced flush' and 'SLM' APIs

[jrodewig]

Regenerates docs to include the following changes from Elasticsearch documentation:

• Synced flush API link change: [DOCS] Separate and reformat synced flush API docs elasticsearch#46634
• SLM API link change: [DOCS] Add redirect for SLM API docs elasticsearch#46838

Plan to backport to 7.x.

[jrodewig]

Blocks elastic/elasticsearch#46875

[jrodewig]

@delvedor You mind taking a look at this when convenient?

It's currently blocking elastic/elasticsearch#46875.

[delvedor]

Hello, sorry for the delay! I can merge this pr, but you are changing code that I'm generating from the rest-api-spec.
The best way to fix this is to update the spec and then regenerate the client code.

[jrodewig]

Hi @delvedor . The changes in this PR were created by running node scripts/generate as outlined in:
https://github.com/elastic/elasticsearch-js/blob/master/CONTRIBUTING.md#code-generation

Here are the updates to the original spec:

• Synced flush: https://github.com/elastic/elasticsearch/blame/master/rest-api-spec/src/main/resources/rest-api-spec/api/indices.flush_synced.json#L4
• GET SLM stats: https://github.com/elastic/elasticsearch/blame/master/x-pack/plugin/src/test/resources/rest-api-spec/api/slm.get_stats.json#L4

Let me know if I missed something.

[delvedor]

Brilliant, thank you!