Improve the comment on `Message::from_digest` #85

Job	Run time
Check for changes to the public API	1m 23s
Test - beta toolchain	41s
Test 32-bit version	28s
Cross test	1m 13s
Test - MSRV toolchain	5m 33s
Test - nightly toolchain	3m 42s
Test - stable toolchain	3m 41s
WASM	1m 59s
	18m 40s

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