Add documentation on run loops #3025

Job	Run time
Check formatting	12s
Check MSRV	46s
Lint macOS 64bit	2m 34s
Lint iOS 32bit	2m 5s
Lint GNUStep + exceptions	1m 27s
Lint header-translator	28s
Lint test-assembly	26s
Test macOS 12	3m 9s
Assembly tests	48s
${{ matrix.name }}	0s
Check framework features	0s
Verify header translator output	57s
Test Compiler-RT	24s
Test GNUStep with libobjc2 v1.9	1m 42s
Test GNUStep with libobjc2 v2.0	1m 8s
Compiler UI	1m 37s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	1m 3s
Test GNUStep 32bit	1m 2s
	19m 48s

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