For types that live in a difference assembly than the one that ships it, make sure the MSBuild task will move them to the appropriate intellisense xml

For example, String lives in System.Private.CoreLib.xml, move its docs to System.Runtime.xml

Start at the MSBuild task that processes IntelliSense XML and trace how it chooses the destination assembly. Use the String example to verify that documentation from System.Private.CoreLib.xml is moved to System.Runtime.xml. Done means types whose implementation assembly differs from their shipping assembly appear in the appropriate IntelliSense XML.

Written by the indexing model from the issue text.