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.
I don't understand this statement.  Does it mean:
```
scala> val pf: PartialFunction[Int, String] = { case 0 => "zero" }
pf: PartialFunction[Int,String] = <function1>

scala> pf.isDefinedAt(1)
res0: Boolean = false
```
I think the phrase for curls and cases is:

http://www.scala-lang.org/files/archive/spec/2.11/08-pattern-matching.html#pattern-matching-anonymous-functions