Include trailing commas in wrapped function declarations [RustDoc]

[scottmcm]

Take a look at the docs for mir::Visitor: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/visit/trait.Visitor.html#method.visit_local

Rustdoc is making good choices for whether to wrap the declarations

But when it wraps, it doesn't include a trailing comma on the last parameter.

I think it should, because the default style guide wants a comma there.

(And rustdoc should continue not putting a trailing comma when the declaration is shown as a single line.)

[Sergi-Ferrez]

@rustbot claim