docs(allocator/hashmap): add comments to HashMap::from_iter_in

[overlookmotel]

Follow-on after #14211. Add comments reflecting conversation we had on that PR.

[overlookmotel]

• docs(allocator/hashmap): add comments to HashMap::from_iter_in #14329 👈 (View in Graphite)
• main

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[Copilot]

Pull Request Overview

This PR adds comprehensive documentation comments to the HashMap::from_iter_in method in the allocator crate, explaining the implementation decisions and trade-offs made during iterator-based HashMap construction.

Key Changes

• Added detailed comments explaining the use of the iterator's lower size bound for capacity estimation
• Documented the trade-offs between potential reallocation vs over-allocation
• Added comments explaining the strategic placement of ManuallyDrop wrapper

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #14329 will not alter performance

_{Comparing 10-04-docs_allocator_hashmap_add_comments_to_hashmap_from_iter_in_ (0c14e50) with main (7a1c339)}

Summary

✅ 37 untouched

[overlookmotel]

Merge activity

• Oct 4, 9:14 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Oct 4, 9:14 AM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Oct 4, 9:21 AM UTC: Merged by the Graphite merge queue.