Add basic documentation for with_capacity

src/libcore/vec.rs

@@ -173,6 +173,7 @@ pub fn from_slice<T:Copy>(t: &[T]) -> ~[T] {
     from_fn(t.len(), |i| t[i])
 }
 
+/// Creates a new vector with a capacity of `capacity`
 pub fn with_capacity<T>(capacity: uint) -> ~[T] {
     let mut vec = ~[];
     reserve(&mut vec, capacity);