[Proposal]: Implementation specific documentation about csharplang

[BillWagner]

@CyrusNajmabadi Does this capture your thoughts on Wednesday?

[BillWagner]

Another area for discussion on this issue:

How do we want to handle spec deviations where the rolsyn compiler behavior enables code constructs that the Standard prohibits.

One example is how roslyn handles reference parameters for iterator methods and async methods

In both clauses, the standard states:

When a function member is implemented using an iterator block (or async method), it is a compile-time error for the formal parameter list of the function member to specify any in, out, or ref parameters, or any parameter of a ref struct type.

However, the roslyn implementation allows this as long as that parameter isn't captured by the iterator or async state machine.