Clarify docs for canConvert(Object, Class)

It was misleading the way it was because I could return true from
canConvert("5.1", int.class) but would get an exception when I actually
tried convert("5.1", int.class). The documentation led me to believe
the check would catch this scenario when it is actually a convenience
method that uses canConvert(Class, Class)
Merge pull request #38 from jaywarrick/conversion-utils-fixes

Clarify docs for canConvert(Object, Class)