Improve enrollment token docs and terminology consistency

[kilfoyle]

From @jillguyonnet in elastic/kibana#191585 (comment):

I believe, however, that some improvements could also be brought to the documentation itself in order to fully address the above concerns. The Fleet enrollment tokens doc page already describes enrollment token creation and deletion, perhaps the following information could be added:

Why the token name must be unique.
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.
A final note around the variations in terminology (not a big issue, I think):

"enrollment tokens" in the docs vs. "enrollment API keys" in the OpenAPI spec (I thought it would be best to keep the latter consistent)
"revoke token" vs. "invalidate API key"

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For the terminology, the docs need to match the UI, so perhaps the best thing would be just to add a note on the Fleet enrollment tokens doc page explaining that enrollment token = enrollment API key.

For some other details see this comment: elastic/kibana#155550 (comment)