Update APIs for v3.0 when released

[farzonl]

open-xml-docs/docs/how-to-insert-a-comment-into-a-word-processing-document.md

Line 99 in c354f7b

do this, call the [MainDocumentPart](https://msdn.microsoft.com/library/office/documentformat.openxml.packaging.maindocumentpart.aspx) generic method, **GetPartsCountOfType**, and specify a kind of **WordprocessingCommentsPart**.

GetPartsCountOfType has been removed from the sdk in the 3.0 beta. This example will need to be updated.

For more context see: dotnet/Open-XML-SDK#1442

[twsouthwick]

The api refs are in a different repo, so adding a link here but we'll track it in this repo: https://github.com/OfficeDev/open-xml-docs-ref-dotnet

[twsouthwick]

This has been updated!