improve condition_variable documentation #2459

Job	Run time
downstream	45m 12s
linux-compat (manylinux1-x64)	1m 7s
linux-compiler-compat (clang-3)	1m 50s
linux-compiler-compat (clang-6)	5m 59s
linux-compat (manylinux1-x86)	1m 16s
linux-compiler-compat (clang-8)	6m 22s
linux-compat (manylinux2014-x64)	1m 33s
linux-compiler-compat (clang-9)	6m 12s
linux-compat (manylinux2014-x86)	1m 21s
linux-compiler-compat (clang-10)	1m 48s
linux-compat (raspbian-bullseye)	6m 40s
linux-compiler-compat (clang-11)	1m 53s
linux-compat (fedora-34-x64)	1m 25s
linux-compiler-compat (gcc-4.8)	1m 58s
linux-compat (opensuse-leap)	1m 25s
linux-compat (rhel8-x64)	1m 19s
linux-compiler-compat (gcc-5)	1m 35s
linux-compat (al2-x64)	1m 40s
linux-compiler-compat (gcc-6)	1m 43s
linux-compiler-compat (gcc-7)	1m 37s
linux-compiler-compat (gcc-8)	1m 44s
linux-compiler-compat (gcc-11)	2m 40s
clang-sanitizers (,thread)	2m 34s
clang-sanitizers (,address,undefined)	1m 50s
linux-no-cpu-extensions	1m 32s
linux-shared-libs	1m 32s
windows	2m 54s
windows-vc16 (x86)	1m 45s
freebsd	9m 0s
windows-vc16 (x64)	2m 16s
macos	1m 20s
macos-no-cpu-extensions	1m 22s
macos-x64	2m 42s
openbsd	11m 49s
windows-app-verifier	21m 15s
windows-no-cpu-extensions	2m 32s
windows-shared-libs	2m 26s
windows-vc14 (x86)	1m 39s
windows-vc14 (x64)	1m 48s
windows-vc15 (x86)	2m 17s
windows-vc15 (x64)	2m 8s
Cross Compile linux-armv6	1m 51s
Cross Compile linux-armv7	1m 58s
Cross Compile linux-arm64	1m 41s
Cross Compile android-armv7	2m 14s
linux-debug	1m 42s
macos-debug	1m 21s
windows-debug	2m 29s
	3h 6m 16s

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