Document PSE fork features #773

Job	Run time
Bitrot check	5m 8s
Build target wasm32-unknown-unknown	6m 58s
Build target wasm32-wasi	3m 57s
Test on ubuntu-latest with basic features	15m 11s
Intra-doc links	3m 58s
Test on windows-latest with basic features	22m 49s
Test on macOS-latest with basic features	16m 34s
Test on ubuntu-latest with all features	20m 23s
Rustfmt	13s
Test on windows-latest with all features	13m 33s
Test on macOS-latest with all features	18m 40s
	2h 7m 24s

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