elaborate on slice wide pointer metadata

[RalfJung]

As decided in https://github.com/rust-lang/unsafe-code-guidelines/issues/510

[traviscross]

@rust-lang/opsem / @RalfJung: In looking carefully at this on the lang-docs call, we realized it might be helpful, both to us and when the lang teams takes this up, if someone might be able annotate the changes here (e.g. using the GH review features), with some explanation for each of these (e.g. "this was incorrect because...", "this was already true because we had said...", "we're adding this guarantee here because...").

[RalfJung]

Note that the PR has two commits; it may help to consider them separately.

[scottmcm]

Starting a lang RFC so it's officially approved, and we can use that to update docs elsewhere following this.

@rfcbot fcp merge

[rfcbot]

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• [ ] @joshtriplett
• [x] @nikomatsakis
• [ ] @pnkfelix
• [x] @scottmcm
• [x] @tmandry
• [x] @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns. See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot reviewed

[rfcbot]

:bell: This is now entering its final comment period, as per the review above. :bell:

psst @scottmcm, I wasn't able to add the final-comment-period label, please do so.

[traviscross]

@rustbot labels -I-lang-nominated +final-comment-period

This is now in FCP so we can unnominate.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

psst @scottmcm, I wasn't able to add the finished-final-comment-period label, please do so.