rP0a82ccb8a4b0

AphrontFileResponse: avoid alien usages of setDownload()

Summary:
I noticed that - historically - setDownload() could also be used with
false, true, or other weird values to activate a download filename.
So, this change continues to give full compatibility to PHP 8.1 but
with extra validations.

This also adds a bit of inline documentation to put this more explicit.

For more context see the previous version:

{5952b0a31b6aac0718bc23aefe43560b9bfe8cc5}

{96ae4ba13acbf0e2f8932e950a92af0495f034d7}

Ref…