docs(ffi): add more documentation for the FFI API (#3424) #2385

Job	Run time
Check Style	16s
Verify hyper.h is up to date	27s
Test C API (FFI)	40s
features	33s
minimal-versions	29s
Test with Miri	1m 12s
Check MSRV	34s
udeps	27s
Test stable on ubuntu-latest	1m 16s
Test stable on windows-latest	2m 15s
Test stable on macOS-latest	1m 26s
Test beta on ubuntu-latest	1m 28s
Test beta on windows-latest	2m 50s
Test beta on macOS-latest	2m 34s
Test nightly on ubuntu-latest	1m 34s
Test nightly on windows-latest	2m 53s
Test nightly on macOS-latest	3m 19s
Check exposed types	20s
Build docs	27s
CI is green	1s
	25m 1s

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