update identifier naming warnings to give an example

This updates identifier warnings such as ``struct `foo_bar` should have a
camel case identifier`` to provide an example.

Closes #14738.
auto merge of #14740 : P1start/rust/name-warnings, r=alexcrichton

This updates identifier warnings such as ``struct `foo_bar` should have a
camel case identifier`` to provide an example.

Closes #14738.