Guide: install instructions
fix feedback from @huonw
thanks @sfackler
Add extra ignore directives.
auto merge of #15162 : steveklabnik/rust/install_instructions, r=brson

This has my voice *very strongly*. I'm not sure if it's too much. I'd find it okay if I had to tone it back, and I don't want it to be _too strong_, but clinical docs are boring.