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.
Thanks, I had an issue with the word usage, too.  Also, is the following clearer?
```
... because if `isDefinedAt` is false, it is not guaranteed that `apply` will throw an exception to indicate an error condition.  

If an exception is not thrown, evaluation may result in an arbitrary value.
```
That wording makes it clear what assumption is contradicted.