Add some documentation in the policy module #134

Job	Run time
Test (macos, macos-latest, stable)	29s
Test (stable, ubuntu-latest, stable)	22s
Test (beta, ubuntu-latest, beta)	25s
Test (mingw, windows-latest, stable-x86_64-pc-windows-gnu)	58s
Test (win32, windows-latest, stable-i686-pc-windows-msvc)	58s
Test (nightly, ubuntu-latest, nightly)	24s
Check Format and Clippy	19s
Minimal versions	19s
Check Documentation	13s
Test (win64, windows-latest, stable-x86_64-pc-windows-msvc)	2m 18s
	6m 45s

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