[Meta] Failure store REST API documentation

[gmarouli]

Description

In this issue we will keep track the REST APIs which docs need to be updated to document the new failure_store query param.

• [ ] GET {index}: include failure store indices by default
• [ ] GET {index}/_stats: default include failure store indices
• [ ] GET _cat/indices: default include failure store indices
• [ ] GET {index}/_settings: default exclude failure store indices
• [ ] GET {index}/_mapping: default exclude failure store indices

[elasticsearchmachine]

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

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)