doc: mention heap allocation earlier in String docs

[JaniM]

Just a tiny addition.

Helps with #123263.

[rustbot]

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[workingjubilee]

I would find this slightly better if it was incorporated into the previous paragraph, to explain that the buffer is indeed what it owns. But at minimum it should distinguish that String is still just a lightweight "smart pointer" that manages a buffer and not the buffer per se.

[workingjubilee]

Oh yes that's much better!

@bors r+

[bors]

📌 Commit 418535b has been approved by workingjubilee

It is now in the queue for this repository.

[workingjubilee]

@bors rollup

[bors]

⌛ Testing commit 418535b with merge 66de611...

[bors]

☀️ Test successful - checks-actions
Approved by: workingjubilee
Pushing 66de611 to master...

[rust-timer]

Finished benchmarking commit (66de611): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 667.771s -> 667.526s (-0.04%)
Artifact size: 315.70 MiB -> 315.67 MiB (-0.01%)