Rollup merge of #36396 - athulappadan:Default-docs, r=bluss

Documentation of what Default does for each type

Addresses #36265
I haven't changed the following types due to doubts:

1)src/libstd/ffi/c_str.rs
2)src/libcore/iter/sources.rs
3)src/libcore/hash/mod.rs
4)src/libcore/hash/mod.rs
5)src/librustc/middle/privacy.rs

r? @steveklabnik

Author: GuillaumeGomez

src/liballoc/arc.rs

@@ -718,6 +718,7 @@ impl<T: ?Sized> Clone for Weak<T> {
 
 #[stable(feature = "downgraded_weak", since = "1.10.0")]
 impl<T> Default for Weak<T> {
+    /// Constructs a new `Weak<T>` without an accompanying instance of T.
     fn default() -> Weak<T> {
         Weak::new()
     }
@@ -923,6 +924,7 @@ impl<T: ?Sized> fmt::Pointer for Arc<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Arc<T> {
+    /// Creates a new `Arc<T>`, with the `Default` value for T.
     fn default() -> Arc<T> {
         Arc::new(Default::default())
     }

src/liballoc/boxed.rs

@@ -290,6 +290,7 @@ impl<T: ?Sized> Box<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Box<T> {
+    /// Creates a `Box<T>`, with the `Default` value for T.
     fn default() -> Box<T> {
         box Default::default()
     }

src/liballoc/rc.rs

@@ -870,6 +870,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
 
 #[stable(feature = "downgraded_weak", since = "1.10.0")]
 impl<T> Default for Weak<T> {
+    /// Creates a new `Weak<T>`.
     fn default() -> Weak<T> {
         Weak::new()
     }

src/libcollections/binary_heap.rs

@@ -263,6 +263,7 @@ impl<T: Clone> Clone for BinaryHeap<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Default for BinaryHeap<T> {
+    /// Creates an empty `BinaryHeap<T>`.
     #[inline]
     fn default() -> BinaryHeap<T> {
         BinaryHeap::new()

src/libcollections/borrow.rs

@@ -249,6 +249,7 @@ impl<'a, B: ?Sized> Default for Cow<'a, B>
     where B: ToOwned,
           <B as ToOwned>::Owned: Default
 {
+    /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
     fn default() -> Cow<'a, B> {
         Owned(<B as ToOwned>::Owned::default())
     }

src/libcollections/btree/map.rs

@@ -1667,6 +1667,7 @@ impl<K: Hash, V: Hash> Hash for BTreeMap<K, V> {
 }
 
 impl<K: Ord, V> Default for BTreeMap<K, V> {
+    /// Creates an empty `BTreeMap<K, V>`.
     fn default() -> BTreeMap<K, V> {
         BTreeMap::new()
     }

src/libcollections/btree/set.rs

@@ -674,6 +674,7 @@ impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BTreeSet<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Default for BTreeSet<T> {
+    /// Makes an empty `BTreeSet<T>` with a reasonable choice of B.
     fn default() -> BTreeSet<T> {
         BTreeSet::new()
     }

src/libcollections/linked_list.rs

@@ -164,6 +164,7 @@ impl<T> LinkedList<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for LinkedList<T> {
+    /// Creates an empty `LinkedList<T>`.
     #[inline]
     fn default() -> Self {
         Self::new()

src/libcollections/string.rs

@@ -1567,6 +1567,7 @@ impl_eq! { Cow<'a, str>, String }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Default for String {
+    /// Creates an empty `String`.
     #[inline]
     fn default() -> String {
         String::new()

src/libcollections/vec.rs

@@ -1652,6 +1652,7 @@ impl<T> Drop for Vec<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Vec<T> {
+    /// Creates an empty `Vec<T>`.
     fn default() -> Vec<T> {
         Vec::new()
     }