Provide documentation for Default impls

For the most part, the `Default` impls are pretty obvious as far as what
we expect them to return, but I think it's better to explicitly state
what's going to be returned in the documentation instead of relying on
the user to make assumptions/look at the source.

Author: frewsxcv

src/liballoc/arc.rs

@@ -606,6 +606,7 @@ impl<T: fmt::Debug> fmt::Debug for Arc<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default + Sync + Send> Default for Arc<T> {
+    /// Creates a new `Arc` using `Arc::new` with the `Default` value of `T`
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> Arc<T> { Arc::new(Default::default()) }
 }

src/liballoc/boxed.rs

@@ -150,12 +150,14 @@ pub unsafe fn into_raw<T : ?Sized>(b: Box<T>) -> *mut T {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Box<T> {
+    /// Creates a new `Box<T>` by `box`ing the `Default` value of `T`
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> Box<T> { box Default::default() }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Box<[T]> {
+    /// Creates a new `Box<[T]>` by `box`ing an empty slice
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
 }

src/libcollections/binary_heap.rs

@@ -171,6 +171,7 @@ pub struct BinaryHeap<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Default for BinaryHeap<T> {
+    /// Creates a new `BinaryHeap` using `BinaryHeap::new`
     #[inline]
     fn default() -> BinaryHeap<T> { BinaryHeap::new() }
 }

src/libcollections/bit.rs

@@ -918,6 +918,7 @@ impl BitVec {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for BitVec {
+    /// Creates a new `BitVec` using `BitVec::new`
     #[inline]
     fn default() -> BitVec { BitVec::new() }
 }
@@ -1128,6 +1129,7 @@ pub struct BitSet {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for BitSet {
+    /// Creates a new `BitSet` using `BitSet::new`
     #[inline]
     fn default() -> BitSet { BitSet::new() }
 }

src/libcollections/btree/map.rs

@@ -863,6 +863,7 @@ impl<K: Hash, V: Hash> Hash for BTreeMap<K, V> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: Ord, V> Default for BTreeMap<K, V> {
+    /// Creates a new `BTreeMap` using `BTreeMap::new`
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> BTreeMap<K, V> {
         BTreeMap::new()

src/libcollections/btree/set.rs

@@ -512,6 +512,7 @@ impl<T: Ord> Extend<T> for BTreeSet<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Default for BTreeSet<T> {
+    /// Creates a new `BTreeSet` using `BTreeSet::new`
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> BTreeSet<T> {
         BTreeSet::new()

src/libcollections/linked_list.rs

@@ -212,6 +212,7 @@ impl<T> LinkedList<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for LinkedList<T> {
+    /// Creates a new `LinkedList` using `LinkedList::new`
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> LinkedList<T> { LinkedList::new() }

src/libcollections/string.rs

@@ -811,6 +811,7 @@ impl Str for String {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for String {
+    /// Creates a new `String` using `String::new`
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> String {

src/libcollections/vec.rs

@@ -1599,6 +1599,7 @@ impl<T> Drop for Vec<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Vec<T> {
+    /// Creates a new `Vec` using `Vec::new`
     #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> Vec<T> {
         Vec::new()

src/libcollections/vec_deque.rs

@@ -83,6 +83,7 @@ impl<T> Drop for VecDeque<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for VecDeque<T> {
+    /// Creates a new `VecDeque` using `VecDeque::new`
     #[inline]
     fn default() -> VecDeque<T> { VecDeque::new() }
 }