Add more documentation for the FFI API #2373

Job	Run time
Verify hyper.h is up to date	17s
Check Style	11s
features	23s
Test with Miri	46s
minimal-versions	24s
Test C API (FFI)	20s
Check MSRV	21s
Test stable on ubuntu-latest	1m 12s
udeps	23s
Test stable on windows-latest	2m 6s
Test stable on macOS-latest	1m 33s
Test beta on ubuntu-latest	1m 7s
Test beta on windows-latest	2m 4s
Test beta on macOS-latest	2m 15s
Test nightly on ubuntu-latest	1m 11s
Test nightly on windows-latest	1m 57s
Test nightly on macOS-latest	2m 13s
Build docs	19s
Check exposed types	17s
CI is green	0s
	19m 19s

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