The ref keyword topic is not complete

[pkulikov]

Related to #5682, ref keyword topic doesn't mention ref struct and related ref readonly struct usage of the ref keyword.

[BillWagner]

In addition to the ref topic, the struct topic should include information on ref struct and ref readonly struct.

[pkulikov]

@BillWagner what about the following structure. The main article on struct should describe:

• "canonical" structs
• readonly struct
• ref struct
• ref readonly struct

Given that, the ref and readonly topics can then only mention usage and link the main article on struct for more information.

The question is: what is the main article on structs? The struct keyword topic references the one in the programming guide:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/structs

Should that page accumulate all information about structs and all other pages link to it?

That rearrangement, if it makes sense, might take several PRs.

[BillWagner]

That makes sense @pkulikov I started updating the ref page, thinking I could fix this issue in #6086 That would make the PR too large to reasonably review. This issue will be fixed in a separate PR.

[BillWagner]

@pkulikov 's list is the right outline.

See #4441 for an important question. To answer that as well, the coverage of readonly struct s must include when the compiler disallows an operation, and when it enables the operation on a defensive copy. For example, what happens when a readonly struct contains a (not readonly) struct? In particular, can methods of the contained struct be called? Are those methods called on defensive copies?
What happens when the not readonly struct is passed by ref (not ref readonly)?