[DOC] unclear/missing documentation for settings sources

[rursprung]

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hidden under the Updating cluster settings using the API chapter it lists the precedence of settings sources. however, (a) this list is incomplete (it's at least missing the environment variables and -E arguments passed at startup) and (b) this should be its own heading (as it's not related to the API).

furthermore, the page is missing an actual example of using environment variables since you can't normally define an env. variable in a shell with dots in it:

$ export discovery.type=single-node
bash: export: `discovery.type=single-node': not a valid identifier

if there's a way to see where which setting came from then this should also be documented (though i don't know if that already exists - if not i guess that'd be a feature request of its own for OpenSearch as it'd be really useful when debugging issues 🙂)

furthermore missing is an information on how to unset a setting which is set at a lower level - e.g. if a setting is set in opensearch.yml, how can one remove that value with an env variable? (for scalar values you can probably just set it to an empty value, but what about list values?)

Version:
all

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Slack discussion