Improve documentation. (#456) #430

Job	Run time
ubuntu_latest_cmake	4m 9s
ubuntu_latest_autotools	13m 35s
macos_latest_cmake	1m 23s
ubuntu_latest_autotools_ilp64	1m 12s
ubuntu_latest_autotools_install	17m 35s
ubuntu_latest_cmake_install	11m 17s
ubuntu_latest_cmake_python	9m 33s
macos_latest_autotools	1m 25s
macos_latest_cmake_python	6m 20s
windows_latest_cmake	2m 16s
	1h 8m 45s

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