rustdoc: Correctly name lifetimes in bounds

Fix #16518
rustdoc: Support unboxed fn sugar in bounds
rustdoc: Remove dummy UnknownBound variant
rustdoc: Include lifetimes in re-exported bounds

Fix #17818
auto merge of #17798 : tomjakubowski/rust/rustdoc-fix-bounds, r=alexcrichton

This PR adds support in rustdoc for properly naming lifetimes in bounds, instead of just showing `'static` for everything. It also adds support for unboxed function sugar bounds, which were also previously rendered as `'static`.
Update COPYRIGHT to better reflect the current repo
auto merge of #17692 : brson/rust/copywut, r=alexcrichton
Rename RELEASES.txt to RELEASES.md. It's markdown.
Update RELEASES.md for 0.12.0
Update AUTHORS.txt for 0.12.0
Update per feedback
Update html_root_url for 0.12.0 release