Add comment around opaque types. #2293

Job	Run time
Format	6s
Clippy	28s
Examples Rust nightly	3m 41s
Docs	1m 7s
Examples Rust beta	3m 42s
Test forcing wrapper generation	8m 8s
Examples Rust stable	3m 46s
Outdated	7s
Examples macOS	4m 12s
Address Sanitizer	19m 11s
Examples Windows (gnu)	6m 41s
Test Rust nightly	9m 54s
Examples Windows (msvc)	5m 13s
Test Rust beta	10m 55s
Test Rust stable	11m 54s
Test macOS	12m 40s
Test Windows (msvc)	20m 29s
	2h 2m 14s

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