Generic structs should include field listings

[andrewrk]

Example: std.ArrayHashMapUnmanaged

It does not list the struct fields, but it should.

I think it should say "Generic Struct" instead of "Type Function" for the main heading.

Related:

• #19249

[nektro]

note to implementors that https://ziglang.org/documentation/master/std/#std.meta.Int is also a "Type Function"