Document PSE fork features #775

Job	Run time
Bitrot check	8m 40s
Build target wasm32-unknown-unknown	7m 18s
Intra-doc links	3m 59s
Build target wasm32-wasi	6m 1s
Rustfmt	15s
Test on ubuntu-latest with basic features	9m 14s
Test on windows-latest with basic features	18m 0s
Test on macOS-latest with basic features	14m 46s
Test on ubuntu-latest with all features	18m 20s
Test on windows-latest with all features	14m 10s
Test on macOS-latest with all features	14m 45s
	1h 55m 28s

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