updated is_avalanching trait protocol

doc/unordered/changes.adoc

@@ -10,6 +10,7 @@
 
 * Added container `pmr` aliases when header `<memory_resource>` is available. The alias `boost::unordered::pmr::[container]` refers to `boost::unordered::[container]` with a `std::pmr::polymorphic_allocator` allocator type.
 * Equipped open-addressing and concurrent containers to internally calculate and provide statistical metrics affected by the quality of the hash function. This functionality is enabled by the global macro `BOOST_UNORDERED_ENABLE_STATS`.
+* Avalanching hash functions must now be marked via an `is_avalanching` typedef with an embedded `value` constant set to `true` (typically, defining `is_avalanching` as `std::true_type`). `using is_avalanching = void` is deprecated but allowed for backwards compatibility.
 
 == Release 1.85.0
 

doc/unordered/hash_quality.adoc

@@ -33,7 +33,7 @@ follows:
 ----
 struct my_string_hash_function
 {
-  using is_avalanching = void; // instruct Boost.Unordered to not use post-mixing
+  using is_avalanching = std::true_type; // instruct Boost.Unordered to not use post-mixing
 
   std::size_t operator()(const std::string& x) const
   {

doc/unordered/hash_traits.adoc

@@ -32,9 +32,14 @@ large changes in the returned hash code —ideally, flipping one bit in the
 the input value results in each bit of the hash code flipping with probability 50%. Approaching
 this property is critical for the proper behavior of open-addressing hash containers.
 
-`hash_is_avalanching<Hash>::value` is `true` if `Hash::is_avalanching` is a valid type,
-and `false` otherwise.
-Users can then declare a hash function `Hash` as avalanching either by embedding an `is_avalanching` typedef
+`hash_is_avalanching<Hash>::value` is:
+
+ * `false` if `Hash::is_avalanching` is not present,
+ * `Hash::is_avalanching::value` if this is present and convertible at compile time to a `bool`,
+ * `true` if `Hash::is_avalanching` is `void` (this usage is deprecated).
+ 
+The behavior is undefined if none of the three cases above is met.
+Users can then declare a hash function `Hash` as avalanching either by embedding an appropriate `is_avalanching` typedef
 into the definition of `Hash`, or directly by specializing `hash_is_avalanching<Hash>` to a class with
 an embedded compile-time constant `value` set to `true`.