Mlbileschi update partial function documentation

To include the fact that the caller is responsible for checking 'isDefinedAt', except when using partial functions defined using {} and case statements.
Hi all,
The following is an example of the type of thing I think the documentation should encourage people to avoid (and this is what I am trying to get at):

val isEvenCase: PartialFunction[Int, String] = {
  case x if x % 2 == 0 => x+" is even"
}

val isEvenInterfaceImpl: PartialFunction[Int, String] = new PartialFunction[Int, String] {
  override def isDefinedAt(x: Int): Boolean = x % 2 == 0
  override def apply(x: Int): String = x+" is even"
}

The second gives isEvenInterfaceImpl(3) = "3 is even", while the first throws a MatchError
I'm not sure that aligns with what @som-snytt was suggesting. Please let me know!