Rollup merge of #141609 - lolbinarycat:core-dedup-ptr-docs-139190, r=workingjubilee

core: begin deduplicating pointer docs

this also cleans up two inconsistancies:
1. both doctests on the ::add methods were actually calling the const version.
2. on of the ::offset methods was missing a line of clarification.

part of rust-lang/rust#139190

Author: workingjubilee