Add more documentation for the FFI API #2374

Job	Run time
Check Style	12s
Verify hyper.h is up to date	27s
Test with Miri	51s
Check MSRV	29s
features	26s
Test stable on ubuntu-latest	1m 9s
Test C API (FFI)	19s
Test stable on windows-latest	1m 58s
Test stable on macOS-latest	2m 17s
minimal-versions	33s
udeps	28s
Test beta on ubuntu-latest	1m 11s
Test beta on windows-latest	2m 1s
Test beta on macOS-latest	1m 58s
Test nightly on ubuntu-latest	1m 13s
Test nightly on windows-latest	2m 8s
Test nightly on macOS-latest	1m 44s
Build docs	15s
Check exposed types	19s
CI is green	1s
	19m 59s

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