specify in documentation that findOptimalK() does not output a scCNA object

[aislyn]

before someone finds out the hard way by saving over their processed scCNA object with a measly tibble...

(also it's one of the few functions not outputting a scCNA object)

[darlanminussi]

good point.
findOptimalK now returns the scCNA object being consistent with expected behavior of other functions.
The information is added to the metadata which can be accessed with metadata(scCNA)

fixed in c731bc9