docs: document sys/eventfd (#2450) #80

Job	Run time
macos-aarch64	2m 15s
minver	18s
macos	3m 30s
linux_native_builds (x86_64-unknown-linux-gnu)	2m 53s
linux_native_builds (x86_64-unknown-linux-musl)	2m 38s
rust_stable	1m 22s
rustfmt	20s
tier3 (x86_64-unknown-dragonfly)	58s
tier3 (x86_64-unknown-openbsd)	58s
tier3 (x86_64-unknown-haiku)	55s
redox	37s
cross (arm-unknown-linux-gnueabi)	1m 53s
cross_compiling (aarch64-linux-android)	40s
cross (armv7-unknown-linux-gnueabihf)	1m 43s
cross_compiling (arm-linux-androideabi)	31s
cross (i686-unknown-linux-gnu)	1m 43s
cross_compiling (armv7-linux-androideabi)	36s
cross (i686-unknown-linux-musl)	1m 51s
cross_compiling (i686-linux-android)	55s
cross (mips-unknown-linux-gnu)	1m 45s
cross_compiling (x86_64-linux-android)	33s
cross (mips64-unknown-linux-gnuabi64)	1m 53s
cross_compiling (arm-unknown-linux-musleabi)	41s
cross (mips64el-unknown-linux-gnuabi64)	1m 53s
cross_compiling (x86_64-unknown-fuchsia)	29s
cross (mipsel-unknown-linux-gnu)	1m 55s
cross_compiling (x86_64-unknown-illumos)	26s
cross (powerpc64le-unknown-linux-gnu)	1m 43s
cross_compiling (aarch64-apple-ios, true)	30s
cross_compiling (powerpc64-unknown-linux-gnu)	33s
cross_compiling (s390x-unknown-linux-gnu)	52s
cross_compiling (x86_64-unknown-linux-gnux32)	40s
cross_compiling (x86_64-unknown-netbsd)	26s
	40m 55s

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