Add some documentation in the policy module #141

Job	Run time
Check Documentation	24s
Test (beta, ubuntu-latest, beta)	30s
Test (win64, windows-latest, stable-x86_64-pc-windows-msvc)	1m 24s
Check Format and Clippy	20s
Test (win32, windows-latest, stable-i686-pc-windows-msvc)	1m 10s
Test (macos, macos-latest, stable)	29s
Test (stable, ubuntu-latest, stable)	16s
Test (nightly, ubuntu-latest, nightly)	22s
Test (mingw, windows-latest, stable-x86_64-pc-windows-gnu)	1m 7s
Minimal versions	25s
	6m 27s

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