[MRELEASE-957] Document how you can use multiple identities on same SCM host

**[Karl Heinz Marbaise](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=khmarbaise)** opened **[MRELEASE-957](https://issues.apache.org/jira/browse/MRELEASE-957?redirect=false)** and commented

It is unfortunately hard to find how you can handle multiple identities on same SCM host. This should be documented in a more prominent location on the site and not only mentioned in FAQ with not enough explanations.

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**Affects:** 2.5.3

**Issue Links:**
- [SCM-826](https://issues.apache.org/jira/browse/SCM-826) Allow multiple identities on same SCM host

Start with the existing FAQ and the site's current release documentation, then review SCM-826 to understand the supported multiple-identity behavior. Done means the guidance is moved to a more prominent location and clearly explains how to use multiple identities on the same SCM host.

Written by the indexing model from the issue text.