Rename `tcx.ensure()` to `tcx.ensure_ok()`, and improve the associated docs
This is all based on my archaeology for https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.60TyCtxtEnsure.60.
The main renamings are:
- `tcx.ensure()` → `tcx.ensure_ok()`
- `tcx.ensure_with_value()` → `tcx.ensure_done()`
- Query modifier `ensure_forwards_result_if_red` → `return_result_from_ensure_ok`
Hopefully these new names are a better fit for the *actual* function and purpose of these query call modes.