Tidy up doc declarations

include/beast/http/basic_parser.hpp

@@ -400,7 +400,7 @@ public:
     std::size_t
     write(ConstBufferSequence const& buffers, error_code& ec);
 
-#if ! GENERATING_DOCS
+#if ! BEAST_DOXYGEN
     std::size_t
     write(boost::asio::const_buffers_1 const& buffer,
         error_code& ec);

include/beast/http/detail/basic_parsed_list.hpp

@@ -32,7 +32,7 @@ public:
     using value_type = typename Policy::value_type;
 
     /// A constant iterator to a list element.
-#if GENERATING_DOCS
+#if BEAST_DOXYGEN
     using const_iterator = implementation_defined;
 #else
     class const_iterator;

include/beast/http/message_parser.hpp

@@ -81,7 +81,7 @@ public:
         resolution only if the first argument is not a
         @ref http::header_parser or @ref message_parser.
     */
-#if GENERATING_DOCS
+#if BEAST_DOXYGEN
     template<class... Args>
     explicit
     msesage_parser(Args&&... args);

include/beast/http/read.hpp

@@ -179,7 +179,7 @@ template<
     class DynamicBuffer,
     bool isRequest, bool isDirect, class Derived,
     class ReadHandler>
-#if GENERATING_DOCS
+#if BEAST_DOXYGEN
 void_or_deduced
 #else
 typename async_completion<