Add note about Result to `find_or_last` and `find_or_first` #996

Job	Run time
miri	2m 56s
doc	7s
check	20s
check format	5s
check (--no-default-features)	11s
msrv	52s
check (--no-default-features --features use_alloc)	8s
semver-checks	2m 31s
check (--all-targets --all-features)	26s
test	51s
All checks succeeded	0s
	8m 27s

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