Rollup merge of #122238 - fee1-dead-contrib:builtin-impl-next-solver-dox, r=lcnr

Document some builtin impls in the next solver

This does not cover all builtin impls, but ones that I were able to go over within a cycle.

r? `@lcnr`

Let me know if the place isn't correct for these, or if you'd like me to change how the impls are presented ^^

Author: matthiaskrgr