[rustdoc] Hide impl block containing only private items by default

[jhpratt]

Zulip thread where this was discussed

cc @GuillaumeGomez

@rustbot label +A-docs +A-rustdoc-ui +C-enhancement +T-rustdoc

This is technically a regression, but it doesn't feel appropriate to label it as such.

[jyn514]

Here is an example of a block that should be hidden when --document-private-items is not passed:

pub struct S;

/// Some docs here
impl S {
  fn foo() {}
}