[8.15] Fix up enrollment token docs (backport #1295)

[mergify]

This adds some detail to clear up usage of Fleet enrollment tokens.

Thanks for the suggestions @jillguyonnet. Let me know if I've missed anything.

Closes: #1270

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Why the token name must be unique.

I think it should be enough for users to know just that the names need to be unique. I added:

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Add an explanation around revoking tokens (marked as inactive, will be removed after expiration) with a link to the retention period setting in Elasticsearch.
A mention that inactive tokens are still returned by the GET API and visible in the UI until they are cleaned up.

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"enrollment tokens" in the docs vs. "enrollment API keys" in the OpenAPI spec

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"revoke token" vs. "invalidate API key"

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This is an automatic backport of pull request #1295 done by [Mergify](https://mergify.com).

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