Clarify the difference between FunctionType.extern and FunctionType.imported

[patacca]

It seems that there shouldn't be any difference between an imported and an extern function, however there are currently two different types. This difference originates from the Quokka python bindings where it is introduced the external type.

If I guessed correctly the problem arises because sometimes IDA behaves very differently with imported functions in a PE binary (treating them like data instead of code). More testing is needed.