Add missing docs for the unistd module #661

Job	Run time
macos	2m 11s
linux_native_builds (x86_64-unknown-linux-gnu)	1m 22s
minver	15s
rust_stable	1m 5s
linux_native_builds (x86_64-unknown-linux-musl)	1m 14s
rustfmt	5s
tier3 (x86_64-unknown-dragonfly)	59s
tier3 (x86_64-unknown-openbsd)	1m 3s
tier3 (x86_64-unknown-haiku)	52s
tier3 (i686-unknown-hurd-gnu)	1m 1s
redox	39s
cross_compiling (aarch64-linux-android)	37s
cross (arm-unknown-linux-gnueabi)	1m 34s
cross_compiling (arm-linux-androideabi)	37s
cross (armv7-unknown-linux-gnueabihf)	1m 42s
cross (i686-unknown-linux-gnu)	1m 54s
cross_compiling (armv7-linux-androideabi)	37s
cross (i686-unknown-linux-musl)	1m 52s
cross_compiling (i686-linux-android)	38s
cross (mips-unknown-linux-gnu)	1m 51s
cross_compiling (x86_64-linux-android)	37s
cross (mips64-unknown-linux-gnuabi64)	1m 46s
cross_compiling (arm-unknown-linux-musleabi)	36s
cross_compiling (x86_64-unknown-fuchsia)	33s
cross_compiling (x86_64-unknown-illumos)	27s
cross (mips64el-unknown-linux-gnuabi64)	1m 40s
cross_compiling (aarch64-apple-ios, true)	32s
cross (mipsel-unknown-linux-gnu)	1m 50s
cross_compiling (powerpc64-unknown-linux-gnu)	38s
cross (powerpc64le-unknown-linux-gnu)	1m 49s
cross_compiling (s390x-unknown-linux-gnu)	39s
cross_compiling (x86_64-unknown-linux-gnux32)	41s
cross_compiling (x86_64-unknown-netbsd)	33s
	34m 29s

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