rustdoc: Update the man page

Closes #9622
rustdoc: Detect repeated keys and fix width

When a key is pressed and held, this now does the OS-style repeating after a bit
of a pause. Also fixes the width of search results to be correct (was changed
beforehand and didn't catch this).
rustdoc: Use a BufferedWriter when emitting source

This takes the rendering time of source files for libstd from 12.5s to 0.1s,
turns out write! calls the write function *a lot*
rustdoc: Stop ignoring dox on fields/variants

This is progress towards #9611
rustdoc: Don't ignore dox on impl blocks

Closes #9611