Add some documentation for public types and methods

[victorhurdugaci]

The packages here are totally missing documentation and it is not obvious, without looking at the code, how are they supposed to be used.

For example, is I have:

Configuration config = new Configuration();
config.AddCommandLine(args);

What is the format accepted by the strings in "args" to be valid? There are many possible options: /key value, --key value, --key:value, key=value, etc.

[HaoK]

Not going to get to this for rc2, clearing for retriage...

[HaoK]

Can you take a look and add documentation if we are still missing any?

[HaoK]

e1b7960

[divega]

Filed dotnet/AspNetCore.Docs#1382 to track adding command line argument formats supported in documentation.