Guide: Pointers: cruft removal + minor clarifications

This removes some leftover line-numbering cruft from elided error examples and brings some minor clarifications.

I’m not super happy about the ‘we cannot have two mutable pointers that point to the same memory’ wording (to the best of my understanding we can’t even have one mutable and one immutable), but other attempts to word this were derailing the flow a bit too much.