Fix up enrollment token docs

[kilfoyle]

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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[jillguyonnet]

Looks great, thank you 🚀

If I was to be super nitpicky, I could suggest adding a link to https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-invalidate-api-key.html, for example:

...setting has expired (see Invalidate API key API for details).

I agree with you regarding only mentioning that the token name must be unique. The reason behind it is not 100% clear and there is a possibility it will not always be the case. I've opened elastic/kibana#192469 to capture the open discussion that resulted from the previous doc improvement.

[kilfoyle]

Good idea @jillguyonnet! I've added that link:

[kilfoyle]

P.S., the new commit automatically removed your approval. Sorry. :-)

[jillguyonnet]

Reapproved 🙂 Thanks for the change!

[karenzone]

LGTM