Rollup merge of #44609 - jonhoo:hash-alloc, r=steveklabnik

Mention that HashMap::new and HashSet::new do not allocate

The docs for `HashMap::with_capacity` and `HashSet::with_capacity` already say that
> If `capacity` is 0, the hash map/set will not allocate.
However, the docs for `::new` do not say that the initial capacity is 0, and thus promise that a call to `::new` alone does not allocate. This PR fixes that.

src/libstd/collections/hash/map.rs

@@ -588,6 +588,9 @@ fn insert_hashed_ordered(&mut self, hash: SafeHash, k: K, v: V) {
 impl<K: Hash + Eq, V> HashMap<K, V, RandomState> {
     /// Creates an empty `HashMap`.
     ///
+    /// The hash map is initially created with a capacity of 0, so it will not allocate until it
+    /// is first inserted into.
+    ///
     /// # Examples
     ///
     /// ```

src/libstd/collections/hash/set.rs

@@ -125,6 +125,9 @@ pub struct HashSet<T, S = RandomState> {
 impl<T: Hash + Eq> HashSet<T, RandomState> {
     /// Creates an empty `HashSet`.
     ///
+    /// The hash set is initially created with a capacity of 0, so it will not allocate until it
+    /// is first inserted into.
+    ///
     /// # Examples
     ///
     /// ```