doc: Remove documentation of IRIX as supported platform.
* doc/posix-headers/netdb.texi: Don't mention IRIX specific workarounds.
* doc/posix-headers/pthread.texi: Likewise.
* doc/posix-headers/sys_socket.texi: Likewise.
* doc/posix-headers/wctype.texi: Likewise.
* doc/posix-functions/btowc.texi: Likewise.
* doc/posix-functions/cbrtf.texi: Likewise.
* doc/posix-functions/cbrtl.texi: Likewise.
* doc/posix-functions/copysignf.texi: Likewise.
* doc/posix-functions/exp2.texi: Likewise.
* doc/posix-functions/exp2f.texi: Likewise.
* doc/posix-functions/exp2l.texi: Likewise.
* doc/posix-functions/expm1f.texi: Likewise.
* doc/posix-functions/expm1l.texi: Likewise.
* doc/posix-functions/fabsl.texi: Likewise.
* doc/posix-functions/isnan.texi: Likewise.
* doc/posix-functions/iswblank.texi: Likewise.
* doc/posix-functions/link.texi: Likewise.
* doc/posix-functions/log10l.texi: Likewise.
* doc/posix-functions/log1pf.texi: Likewise.
* doc/posix-functions/log2.texi: Likewise.
* doc/posix-functions/log2f.texi: Likewise.
* doc/posix-functions/log2l.texi: Likewise.
* doc/posix-functions/lseek.texi: Likewise.
* doc/posix-functions/nl_langinfo.texi: Likewise.
* doc/posix-functions/pthread_sigmask.texi: Likewise.
* doc/posix-functions/remainderf.texi: Likewise.
* doc/posix-functions/remainderl.texi: Likewise.
* doc/posix-functions/rintf.texi: Likewise.
* doc/posix-functions/sigaltstack.texi: Likewise.
* doc/posix-functions/strtod.texi: Likewise.
* doc/posix-functions/strtold.texi: Likewise.
* doc/posix-functions/vscanf.texi: Likewise.
* doc/posix-functions/wctob.texi: Likewise.
* doc/**/*.texi: Update.