Document an error case for at_derivation_index() #2039

Job	Run time
Test - stable toolchain (minimal)	16m 59s
Test - 1.63.0 toolchain (minimal)	25m 21s
Test - stable toolchain (recent)	17m 25s
Test - nightly toolchain (minimal)	14m 10s
Test - 1.63.0 toolchain (recent)	24m 16s
Test - nightly toolchain (recent)	14m 5s
Bench - nightly toolchain (recent)	55s
Docs - stable toolchain (recent)	20s
Docs - nightly toolchain (recent)	26s
Embedded	1m 31s
Format - nightly toolchain	11s
Integration tests - stable toolchain (26_0)	51s
Integration tests - stable toolchain (25_2)	55s
Integration tests - stable toolchain (25_1)	1m 0s
Integration tests - stable toolchain (25_0)	48s
Integration tests - stable toolchain (24_2)	52s
Integration tests - stable toolchain (24_1)	48s
Integration tests - stable toolchain (24_0_1)	50s
Integration tests - stable toolchain (23_2)	50s
Integration tests - stable toolchain (23_1)	1m 1s
Integration tests - stable toolchain (23_0)	46s
Lint - nightly toolchain (recent)	31s
Integration tests - stable toolchain (22_1)	55s
Integration tests - stable toolchain (22_0)	57s
Integration tests - stable toolchain (0_21_2)	53s
Integration tests - stable toolchain (0_20_2)	48s
Integration tests - stable toolchain (0_19_1)	55s
Integration tests - stable toolchain (0_18_1)	59s
Integration tests - stable toolchain (0_17_1)	1m 5s
	2h 11m 23s

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