Remove misleading 'rvalue' terminology from transmute docs

[Herrtian]

Summary

Fixes #153350

The transmute documentation example contained the comment:

// This now has three mutable references pointing at the same
// memory. `slice`, the rvalue ret.0, and the rvalue ret.1.

The term "rvalue" is not used elsewhere in standard library documentation and is confusing to readers. Additionally, there is no variable named ret in the code — the function returns an anonymous tuple.

Changed to:

// This now has three mutable references pointing at the same
// memory: `slice`, and the two returned mutable slices.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

r? @jhpratt

rustbot has assigned @jhpratt.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
• @scottmcm, libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, jhpratt, scottmcm

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • 2dfb1f3

[jieyouxu]

Hi, you've opened 10 PRs in very quick succession, and looking at the PR descriptions and some of the tests, I have some doubts regarding if the contributions are adaqutely self-reviewed. Recently, we have seen waves of automated contributions that are not adequately self-reviewed, which is why we are being more cautious.

In my capacity as a venue moderator, I am requesting that you respond to reviewer questions to

• Add regression test for #138710 (interpret queries ICE) #154789
• Add regression test for #132767 (try_normalize_erasing_regions ICE) #154792
• Add regression test for #132055 (escaping bound vars ICE) #154790
• Add regression test for #119316 (closure in const generic default ICE) #154791

I am hereby placing a temporary embargo on creation of new PRs from you before the PRs mentioned above are responded to, and the reviewers develop more confidence that you are self-reviewing and understand your contributions. The temporary embargo will be lifted if the reviewers have sufficient confidence in your contributions.

Before the temporary embargo is lifted, any further PRs from your account will be closed. If new PRs continue to be raised from your account before so, further moderation action may be applied.

I am closing and locking this PR before the review discussions in the PRs linked above are settled and the temporary embargo lifted. If the temporary embargo is lifted, I will unlock and reopen this PR.