Add note about const-ness of with_hasher

Author: mwillsey

src/map.rs

@@ -175,7 +175,12 @@ impl<K, V, S> IndexMap<K, V, S> {
         }
     }
 
-    /// Create a new map with `hash_builder`
+    /// Create a new map with `hash_builder`.
+    ///
+    /// This function is `const`, so it
+    /// can be called in `static` contexts.
+    /// If you need `hash_builder` that can be made in the static context,
+    /// consider [`ahash`](https://docs.rs/ahash/0.7.4/ahash/struct.RandomState.html#method.with_seeds).
     pub const fn with_hasher(hash_builder: S) -> Self {
         IndexMap {
             core: IndexMapCore::new(),

src/set.rs

@@ -155,7 +155,12 @@ impl<T, S> IndexSet<T, S> {
         }
     }
 
-    /// Create a new set with `hash_builder`
+    /// Create a new set with `hash_builder`.
+    ///
+    /// This function is `const`, so it
+    /// can be called in `static` contexts.
+    /// If you need `hash_builder` that can be made in the static context,
+    /// consider [`ahash`](https://docs.rs/ahash/0.7.4/ahash/struct.RandomState.html#method.with_seeds).
     pub const fn with_hasher(hash_builder: S) -> Self {
         IndexSet {
             map: IndexMap::with_hasher(hash_builder),