rust.md: Explicitly point out how special `'static` is.

Drive-by: fix description of `&content` to point out that `&'f type`
is (as of today) only for type expressions.
auto merge of #16381 : pnkfelix/rust/fsk-rust.md-fixes, r=alexcrichton

rust.md: Explicitly point out how special `'static` is.

Drive-by: fix description of `&content` to point out that `&'f type` is (as of today) only for type expressions.