Added docs for std::hash

But I do not know if they copile with QuickBook.

doc/00_optional.qbk

@@ -2,7 +2,7 @@
     [quickbook 1.4]
     [authors [Cacciola Carballal, Fernando Luis]]
     [copyright 2003-2007 Fernando Luis Cacciola Carballal]
-    [copyright 2014-2021 Andrzej Krzemieński]
+    [copyright 2014-2022 Andrzej Krzemieński]
     [category miscellaneous]
     [id optional]
     [dirname optional]

doc/27_ref_optional_synopsis.qbk

@@ -73,6 +73,16 @@
 
     } // namespace boost
 
+    namespace std {
+
+    template <typename T>
+    struct hash<boost::optional<T> > ; ``[link reference_std_hash_spec __GO_TO__]``
+
+    template <typename T>
+    struct hash<boost::optional<T&> > ; ``[link reference_std_hash_spec __GO_TO__]``
+
+    } // namespace std
+
 
 [endsect]
 

doc/28_ref_optional_semantics.qbk

@@ -1408,3 +1408,37 @@ assert (addressof(*opt0) == addressof(y));
 ``
 
 [endsect]
+
+
+[section Detailed Semantics - std::hash Specializations]
+
+__SPACE__
+[#reference_std_hash_spec]
+
+``
+namespace std {
+
+template <typename T>
+struct hash<boost::optional<T> > ;
+
+template <typename T>
+struct hash<boost::optional<T&> > ;
+
+} // namespace std
+``
+
+The specialization `hash<optional<T>>`` is enabled if and only if
+`hash<remove_­const_­t<T>> is enabled. When enabled, for an object `o`
+of type `optional<T>``, if `o.has_­value() == true`, then `hash<optional<T>>()(o)`
+ evaluates to the same value as `hash<remove_­const_­t<T>>()(*o)``; otherwise it
+evaluates to an unspecified value.
+The member functions are not guaranteed to be `noexcept`.
+
+[caution
+You may get compiler errors when your program provides specializations for
+`std::hash<boost::optional<T>>`. If this happens, define macro
+`BOOST_OPTIONAL_CONFIG_DO_NOT_SPECIALIZE_STD_HASH` to suppress the specializations
+of `std::hash` in this library.
+]
+
+[endsect]

doc/91_relnotes.qbk

@@ -1,7 +1,7 @@
 [/
     Boost.Optional
 
-    Copyright (c) 2015 - 2021 Andrzej Krzemienski
+    Copyright (c) 2015 - 2022 Andrzej Krzemienski
 
     Distributed under the Boost Software License, Version 1.0.
     (See accompanying file LICENSE_1_0.txt or copy at
@@ -11,6 +11,10 @@
 
 [section:relnotes Release Notes]
 
+[heading Boost Release 1.80]
+
+* Added specializations for `std::hash<boost::opitonal<T>>`. This fixes [@https://github.com/boostorg/optional/issues/55 issue #55].
+
 [heading Boost Release 1.79]
 
 * Fixed [@https://github.com/boostorg/optional/issues/98 issue #98].