Clarify the scope of index template configuration

[fdartayre]

Related #66163

We could clarify the section in put-index-template.asciidoc:

Creates or updates an index template. Index templates define settings, mappings, and aliases that can be applied automatically to new indices.

For data streams, it's true for settings and mappings, but aliases are created at data stream level and are not applied to backing indices. In particular, modifying aliases has no effect on rollover.

Similarly in index-templates.asciidoc:

An index template is a way to tell Elasticsearch how to configure an index when it is created. For data streams, the index template configures the stream’s backing indices as they are created.

While it's true for settings and mappings, I don't think it is for aliases, which are created at data stream level.

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

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