Porting .NET MSDN docs to Core - Automatic Memory Management

[mairaw]

We need to port the relevant content from MSDN that applies to .NET Core.

The Automatic Memory Management doc is still applicable to .NET Core. This is the topic:
https://msdn.microsoft.com/en-us/library/f144e03t(v=vs.110).aspx

There's also this section on GC: https://msdn.microsoft.com/en-us/library/hh156531(v=vs.110).aspx

[BillWagner]

closed in PR #975