Document `withMainSerialExecutor` caveats #41

Job	Run time
Wasm (wasm-5.7.1-RELEASE)	1m 20s
Windows (windows-latest, debug)	2m 46s
Windows (windows-latest, release)	2m 32s
Linux (ubuntu-20.04, debug)	34s
Linux (ubuntu-20.04, release)	51s
macOS (14.3.1, debug)	2m 30s
macOS (14.3.1, release)	2m 50s
	13m 23s

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