Add articles on record structs

[BillWagner]

C# 10 adds record struct types. These types follow value semantics (as all value types do). The compiler generates many of the same methods as it does for record types. Some aren't applicable because they are only needed to support inheritance for record types.

In addition, record types can be declared as record class types to avoid potential confusion with record struct types.

Feature specification: Record structs

[BillWagner]

To address this issue, the following tasks must be completed:

• Update the language reference on record to include record struct. This should also point out that record class and record are the same type.
• The language reference text on record struct should make it clear which methods are synthesized for record structs, and which are for record classes.
• The Fundamentals article on Records needs to be updated to include record struct types.
• The What's new tutorial on Records needs to be updated to include record struct types.

[BillWagner]

Fix #24681 in the same PR as this issue.

[BillWagner]

Note that now all struct types support with expressions, not just record structs. That ripples through some of our language reference work on struct types.

[pkulikov]

Note that now all struct types support with expressions, not just record structs. That ripples through some of our language reference work on struct types.

@BillWagner #26100 takes care of that.