Use rustdoc or similar to mark all APIs exposed or rexposed from arti-client.

We would like every type and function that is visible from arti-client to get marked with a semver warning, similar to:

/// SEMVER NOTE: This type is exposed from arti-client; do not make changes to it lightly!

We have been doing this when we remember to do so, but it is the sort of thing that it would be good to automate, if possible. See also #711.