rustdoc: Fix hiding of private fields

The calculation for whether a field is public or private was tweaked in #13184,
but I forgot to update rustdoc.

Closes #13310
auto merge of #13319 : alexcrichton/rust/rustdoc-fields, r=brson

The calculation for whether a field is public or private was tweaked in #13184,
but I forgot to update rustdoc.

Closes #13310