rustdoc: Don't strip empty modules with documentation

Closes #11443
doc: build the docs for librustuv

Closes #11444
rustdoc: Get --version working

Closes #11421
rustdoc: Don't show private modules
rustdoc: Don't show a fields header if there are none
auto merge of #11461 : alexcrichton/rust/rustdoc-fixes, r=brson

See the commits.