Minor changes to `rust.md`, and `guide-ffi.md`.

  * `rust.md`: whanges for consistency

  * `guide-ffi.md`: wrapped inline code
auto merge of #16440 : bheesham/rust/master, r=brson

  * `rust.md`: changes for consistency

  * `guide-ffi.md`: wrapped inline code

NOTE: This is a duplicate of #16375. I completely messed up that fork, so I made a new fork.