Improve documentation of struct expressions

[WaffleLapkin]

This compiles:

trait Tr { type T; }
impl<T> Tr for T { type T = T; }

struct Enum { Unit, Tuple() }

<Enum as Tr>::T::Unit; // OK
<Enum as Tr>::T::Tuple(); // OK

So qualified paths can be used when constructing tuple/unit variants. This PR documents that. (note that tuple/unit struct expressions aren't really a thing -- they are just calls/paths, so this doesn't necessarily change much, calls/paths already have correct docs)

Additionally I added more mentions of the fact that these expressions are the same for enums.

[ehuss]

Thanks!

I opened #1802 to capture my thoughts on this in general, and to consider just removing all of this. (Not a blocker for this PR.) I've never been particularly comfortable with how this is presented.

[ehuss]

Maybe to simplify this further it can be StructExprTuple -> CallExpression

[WaffleLapkin]

Done ✅