Rollup merge of #83579 - RalfJung:ptr-arithmetic, r=dtolnay

Rollup merge of #83579 - RalfJung:ptr-arithmetic, r=dtolnay

Improve pointer arithmetic docs

* Add slightly more detailed definition of "allocated object" to the module docs, and link it from everywhere.
* Clarify the "remains attached" wording a bit (at least I hope this is clearer).
* Remove the sentence about using integer arithmetic; this seems to confuse people even if it is technically correct.

As usual, the edit needs to be done in a dozen places to remain consistent, I hope I got them all.