Have rustdoc document private items, except use and extern crate

[SimonSapin]

Rustdoc defaults to only documented things that are public, i.e. things that the user of a library would care about. This is a good default, but rustdoc can also be useful to people working on large crates by documenting these crates themselves. In that case, it’s preferable to also include private types, fields, functions, and methods.
#15347 was about this use case, and was closed with this solution:

rustdoc --no-defaults --passes "collapse-docs unindent-comments"

We just tried this on http://doc.servo.org/ . It’s better than before, but has the unexpected side effect that every use item (including non-pub and the implicit prelude) and every extern crate (including the implicit std) is documented under “Reexports”. It can get pretty noisy:

They’re technically part of the crate, but don’t provide useful information. Could rustdoc split up the strip-private pass into one that strips private use and extern crate, and one for everything else?