proto is gone

The following files need to be adjusted, too (i can't do it now):
- doc/po/ja/rust.md.po
- doc/po/rust.md.pot
Some rustdoc manual fixes

Clarify that it's sundown, and that we don't support magic
auto merge of #10913 : cmr/rust/rustdoc_man, r=huonw

Clarify that it's sundown, and that we don't support magic