Added doc comment w/ links explaining the difference between 32-bit and 64-bit libc for readdir_r
Added doc comment w/ links explaining the difference between 32-bit and 64-bit libc for readdir_r

rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc