Fix and improve the documentation for Swagger UI upgrade

This is regarding the doc https://access.redhat.com/documentation/en-us/red_hat_3scale/2.1/html/api_documentation/activedocs-upgrade-22 (and the same one for SaaS).

Originally was posted in THREESCALE-289, but as the doc is in prod already, opening a new issue.

1) It should be "UPGRADE SWAGGER UI 2.1.3 TO 2.2.10", and wherever it is refered to Swagger UI, it should be `2.1.3` (not active_docs, so active_docs version: "2.0" should remain as it is), as this is the version that was used before the upgrade.

2) Information about how to select a different specification is missing.
See this issue https://issues.jboss.org/browse/THREESCALE-304 and specifically this comment.

The suggested text to add to the bottom of the article:

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The first specification published in APIs > ActiveDocs will be loaded by default.

4. In order to use any ActiveDocs specification, add the following line:

window.swaggerUi.options['url'] = "{{provider.api_specs.<spec_system_name>.url}}";

before the line:

window.swaggerUi.load();

Replace <spec_system_name> with the system name of the specification you want to load.

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