Prefix scale for UTL_MATCH.JARO_WINKLER_SIMILARITY
You are right that the Oracle documentation does not mention a prefix scale factor. This leads me to conclude that what Oracle is offering is a simpler version of the formula where there is no option to choose a prefix length. There is no other Oracle documentation on this that I know of and it looks unchanged from 10g when it was first implemented.
Even if there was a hidden implementation it would not be advisable to use it in production code as there is no guarantee of support or that it will remain available.
From the Wikipedia reference you gave it looks like you could create your own function which implements the Jaro Winkler similarity formula exactly.