[SCM-1015] Clarify difference between ScmManager and ScmProvider

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[SCM-1015](https://issues.apache.org/jira/browse/SCM-1015?redirect=false)** and commented

I was assuming that `ScmProvider` is the SPI while `ScmManager` is the (consumer) API. But there are some methods from `ScmProvider` which are not called from anywhere else (except for test code) like:
* `untag`
* `info`
* `remoteinfo`

Are those just missing from `ScmManager` (and their implementation delegating to the `ScmProvider`'s method in `AbstractScmManager`) or is there no clear separation between `ScmProvider` and `ScmManager`?

At the same time some clarification on differences between `ScmRepository` and `ScmProviderRepository` would be helpful.

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No further details from [SCM-1015](https://issues.apache.org/jira/browse/SCM-1015?redirect=false)

Start by comparing the ScmProvider, ScmManager, and AbstractScmManager entry points, including the untag, info, and remoteinfo methods mentioned in the issue. Then inspect ScmRepository, ScmProviderRepository, and related test code. Done means the roles and separation of these interfaces are clearly documented, including whether the provider methods should be exposed through ScmManager.

Written by the indexing model from the issue text.