Document PSE fork features #776

Job	Run time
Rustfmt	21s
Build target wasm32-unknown-unknown	7m 1s
Bitrot check	11m 7s
Build target wasm32-wasi	3m 59s
Test on ubuntu-latest with basic features	14m 21s
Intra-doc links	6m 53s
Test on windows-latest with basic features	17m 55s
Test on macOS-latest with basic features	14m 16s
Test on ubuntu-latest with all features	10m 41s
Test on windows-latest with all features	13m 48s
Test on macOS-latest with all features	13m 12s
	1h 53m 34s

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