Issue 10506: document how columns are counted in std.string.wrap.

[quickfur]

See issue #10506

This PR at least documents what the code currently does. Enhancements to be done in the future if we deem it necessary.

[dlang-bot]

Thanks for your pull request, @quickfur!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10834"