update crate_universe docs (#778)
update crate_universe docs (#778)

It took me quite a while to figure out how to use it based on the
existing examples, as some files and functions had been renamed, and
I overlooked the call to pinned_rust_install().
bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust