[spec/hash-map] Improve Properties section

[ntrel]

Some tweaks following #4339.

Merge AA Properties and AA Organizing Operations tables.
Add subheading for iteration operations.
Move rehash paragraph into table.
Add link to object docs for examples.
Don't hardcode HTML links - use dlang.org macros.
Tweak wording.
Move byKeyValue paragraph into table.
Use BEST_PRACTICE for iteration.
Add subheading for key lookup operations.
Add missing THEAD.
Improve update description.

[dlang-bot]

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