rustdoc: when hiding item contents, include a count

[jsha]

In #83337, we started hiding the contents (methods, fields, variants) of large traits, structs, and enums. When the contents are hidden there's text that says [+] Show methods. It's a little confusing as a user to figure out why some contents are hidden and some aren't. To improve that, we should include the number as a hint. E.g. [+] Show 15 methods.

/cc @Manishearth

[Manishearth]

Oh this is a neat idea!

[weihanglo]

@rustbot claim