alloc: Document panics when allocations will exceed max

[leftmostcat]

Document panics in String and Vec due to capacity overflowing isize::MAX. Correct outdated claims of usize::MAX limit.

Fixes #148598.

Ping @lolbinarycat

[rustbot]

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[leftmostcat]

At a glance, the build failures appear to be related to CI setup and not to these changes.

[lolbinarycat]

CI is currently broken, it should be fixed once #148665 is merged.

[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.