Rollup merge of #109504 - steffahn:stabilize_a_rc_into_inner, r=joshtriplett

Stabilize `arc_into_inner` and `rc_into_inner`.

Stabilize the `arc_into_inner` and `rc_into_inner` library features and thus close #106894.

The changes in this PR also resolve the FIXMEs for adjusting the documentation upon stabilization, and I’ve additionally included some very minor documentation improvements.

```@rustbot``` label +T-libs-api -T-libs

Author: matthiaskrgr