Document and clean up ty::print some more.

In order to get #58140 merged before it bitrots again, and needs an eleventh rebase, I've left some of the review comments (by @nikomatsakis) unresolved.

There's also a few things, such as const generics, `$(...)?` in macros, etc. that weren't possible when the PR was originally authored, but can be taken into account now.