Extension methods should be documented

[davidjohnoliver]

I'm submitting a...

• Documentation issue or request

Current behavior

Most of the extension methods in Uno.Core (eg here https://github.com/unoplatform/Uno.Core/blob/master/src/Uno.Core/Collections/EnumerableExtensions.cs) lack xml documentation.

Expected behavior

Methods are documented with what they actually do.

Minimal reproduction of the problem with instructions

Environment

Nuget Package: 

Package Version(s): 

Affected platform(s):
- [ ] iOS
- [ ] Android
- [ ] WebAssembly
- [ ] Windows
- [ ] Build tasks

Visual Studio
- [ ] 2017 (version: )
- [ ] 2017 Preview (version: )
- [ ] for Mac (version: )

Relevant plugins
- [ ] Resharper (version: )