Document the 'const char **' vs. 'char **' problem.

Reported by Braden Ganetsky <braden.ganetsky@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gnu-libiconv/2025-05/msg00003.html>.

* man/iconv.3 (SYNOPSIS, CONFORMING TO): List also the POSIX-compatible
declaration. Explain the background. Mention AM_ICONV.
2 files changed