awk: Document deprecated behavior of hex constants and locales.

FreeBSD will convert "0x12" from hex and print it as 18. Other awks will
convert it to 0. This extension has been removed upstream, and will be
removed in FreeBSD 14.0.

FreeBSD used to set the locale on startup, and make the ranges use that
locale. This lead to weird results like "[A-Z]" matching lower case
characters in some locales. This bug has been fixed.

MFC After: 3 days
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