doc: Mention the 64-bit inode number problem. * m4/largefile.m4 (AC_SYS_LARGEFILE): Mention that this macro fixes the 64-bit inode number problem. * doc/posix-functions/stat.texi: Mention that this module fixes the 64-bit inode number problem. * doc/posix-functions/lstat.texi: Likewise. * doc/posix-functions/fstat.texi: Likewise. * doc/posix-functions/readdir.texi: Add more details. * doc/posix-functions/readdir_r.texi: Likewise.