rustdoc: Render private fields in tuple struct as /* private fields */

[GuillaumeGomez]

Reopening of #110552. All that was missing was a test for the different cases so I added it into the second commit.

Description from the original PR:

I've gotten some feedback that the current rustdoc rendering of...

struct HasPrivateFields(_);

...is confusing, and I agree with that feedback, especially compared to the field struct case:

struct HasPrivateFields { /* private fields */ }

So this PR makes it so that when all of the fields of a tuple variant are private, just render it with the /* private fields */ comment. We can't always render it like that, for example when there's a mix of private and public fields.

cc @jsha
r? @notriddle