ecdsa: fix `clippy::missing_errors_doc` lints (#1411) #2530

Job	Run time
build (wasm32-unknown-unknown, stable)	19s
test (1.85.0)	23s
build (thumbv7em-none-eabi, stable)	25s
build (thumbv7em-none-eabi, 1.85.0)	19s
build (wasm32-unknown-unknown, 1.85.0)	18s
test (stable)	22s
	2m 6s

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