Add more details to Upgrade assistant API docs

[LeeDr]

Kibana version: 7.7.0

Elasticsearch version: 7.7.0

Describe the bug: Upgrade assistant API docs should describe more details.
https://www.elastic.co/guide/en/kibana/7.7/upgrade-assistant-api.html

1. They should say that your original index will be deleted at the end of the reindexing and that an alias will be created with the original index name and pointing to the new index.

2. Does the index get locked for writes, and/or reads during the reindex? Maybe the Kibana reindex API uses an underlying Elasticsearch reindex API and these details are covered there? A user might assume they can continue writing and reading from the index during the reindex and that the replacement of the original index with the alias is atomic. I don't know if that's true or not.

3. Our APIs should document what privileges are required. This is a pretty widespread problem and doesn't have to be addressed as part of this issue. This operation might typically be performed by a superuser even if we documented specific privs required.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[cjcenizal]

@yuliacech Could you please review this and close if it's been addressed by #112960? If there are any outstanding details that weren't included in that PR, would you mind addressing them in a followup? Thanks!

[yuliacech]

Closing this issue as we addressed comments 1 and 2 in #112960. The privileges needed to use Upgrade Assistant are also already listed in these docs.