[Feature]: Documentation for CertificateManager

[esbenbach]

Documentation request.
The, at the time, latest release notes for 10.1 CU3 (https://github.com/microsoft/service-fabric/blob/master/release_notes/Service_Fabric_ReleaseNotes_101CU3.md) describe a new feature for working with certificates, the  CertificateManager class.

There is no documentation to be found (or I am blind), and I have not found the class in neither this repository or the "dotnet" repository.

The closest thing I can find is this:

service-fabric/src/prod/src/managed/Api/src/System/Fabric/Common/CertificateManager.cs

Line 14 in 887a7e5

internal class CertificateManager

Which is internal, and as far as I can tell does not cover the scenarios described in the release notes.

Describe the solution you'd like
Some documentation - or lacking that an update of the source in the correct repository so we can see what it is.

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Assignees: /cc @microsoft/service-fabric-triage