doc about some IRIX 5.3 problems.

doc about some IRIX 5.3 problems.

* doc/posix-functions/getpwnam_r.texi: Mention incompatible declaration
on IRIX 5.3.
* doc/posix-headers/poll.texi: Mention missing nfds_t on IRIX 5.3.
* doc/posix-functions/ptsname.texi: Mention missing declaration on IRIX
5.3.
* doc/posix-functions/grantpt.texi: Likewise.
* doc/posix-functions/unlockpt.texi: Likewise.
* doc/posix-functions/lgamma.texi: Likewise.
* doc/posix-functions/nextafter.texi: Likewise.
* doc/posix-functions/remainder.texi: Likewise.
* doc/posix-functions/select.texi: Mention misplaced declaration on
IRIX 5.3.
Reported by Tom G. Christensen <tgc@jupiterrise.com>.