Clean up transmute lints' documentation

[felix91gr]

1. Converted example calls of transmute from core::intrinsics::transmute to std::mem::transmute.
2. Added missing unsafe block for transmute_ptr_to_ref's example
3. Added more context to useless_transmute's example
4. Added a missing space in a doc comment
5. Added MSRV handling to transmute_ptr_to_ref, such that it handles const contexts gracefully. This fixes its known and documented problem of giving the wrong suggestion prior to 1.58 and 1.83 for transmutation of *const and *mut pointers, respectively.

changelog: transmute_ptr_to_ref fixed FP in const contexts, where under low values of MSRV, alternatives to transmutation didn't exist.

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[felix91gr]

Documentation changes don't need changelog lines in the PR description.

View changes since this review

Oh thank god, this is so much better! Thx, Jason!

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

:umbrella: The latest upstream changes (possibly 06ded880d28bbac87affd8fac3ac72a216021b60) made this pull request unmergeable. Please resolve the merge conflicts.