Improve the comment on `Message::from_digest` #77

Job	Run time
Test - MSRV toolchain	2m 15s
Test 32-bit version	56s
Test - beta toolchain	31s
Test - nightly toolchain	29s
Test - stable toolchain	28s
Check for changes to the public API	1m 28s
Cross test	1m 17s
WASM	2m 34s
	9m 58s

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