Document what a "dangling pointer" is

Location

There are several places this applies, e.g.:

• std::ptr::dangling
• std::ptr::from_ref (in the examples)
• dangling_pointers_from_temporaries lint

Summary

We have several place in the standard library or as part of lints where we use the term "dangling pointer" without defining it anywhere.

We should have a place where we define what that is, ideally in the standard library, in it's own section, maybe under std::ptr, so it can be referenced elsewhere.

It should probably have a definition, followed by some bad examples (and maybe how to fix them).

Wikipedia has article on the subject, I don't know if it could serve as a base for our documentation but it may be worth checking it.

Steps

• Add std documentation
• Update existing std docs
• Update lints diagnostics

Start with the linked std::ptr::dangling and std::ptr::from_ref documentation, then inspect the dangling_pointers_from_temporaries lint. Define “dangling pointer” in a reusable standard-library section with bad examples and possible fixes, and update the existing std docs and lint diagnostics listed in the issue.