[fs.norm.ref] Dissolve and follow #4175 suit

[JohelEGP]

After #4175, I think [fs.norm.ref] should be dissolved, POSIX removed from the footnote (as it is earlier in the document), and the rest of the footnote moved to the first mention of Windows(r) (after changing "these products" to "this product").

[zygoloid]

Merged to master in 2976a21.