docs: clarify case insensitive dictionary hosting

[EvanHahn-Signal]

Description of Change

You must either (1) have a case-insensitive file server (2) upload files in both casings. See #22482 (comment). This improves the docs to clarify that.

Checklist

• relevant documentation is changed or added

Release Notes

Notes: none

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[trop]

I have automatically backported this PR to "16-x-y", please check out #32401

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I have automatically backported this PR to "17-x-y", please check out #32402