Add doxygen docs

libs/full/collectives/include/hpx/collectives/argument_types.hpp

@@ -50,11 +50,29 @@ namespace hpx::collectives {
         struct arity_tag;
     }    // namespace detail
 
+    /// The number of participating sites (default: all localities)
     using num_sites_arg = detail::argument_type<detail::num_sites_tag>;
+
+    /// The local end of the communication channel
     using this_site_arg = detail::argument_type<detail::this_site_tag>;
+
+    /// The opposite end of the communication channel
     using that_site_arg = detail::argument_type<detail::that_site_tag>;
+
+    /// The generational counter identifying the sequence number of the
+    /// operation performed on the given base name. It needs to be supplied
+    /// only if the operation on the given base name has to be performed
+    /// more than once. It must be a positive number greater than zero.
     using generation_arg = detail::argument_type<detail::generation_tag>;
+
+    /// The site that is responsible for creating the support object
+    /// of the operation. It defaults to '0' (zero).
     using root_site_arg = detail::argument_type<detail::root_site_tag, 0>;
+
+    /// The tag identifying the concrete operation
     using tag_arg = detail::argument_type<detail::tag_tag, 0>;
+
+    /// The number of children each of the communication nodes is connected
+    /// to (default: picked based on num_sites).
     using arity_arg = detail::argument_type<detail::arity_tag>;
 }    // namespace hpx::collectives

libs/full/collectives/include/hpx/collectives/channel_communicator.hpp

@@ -117,6 +117,8 @@ namespace hpx { namespace collectives {
 
     ///////////////////////////////////////////////////////////////////////////
     // forward declarations
+    /// a handle identifying the communication channel to use for get/set
+    /// operations
     class channel_communicator;
 
     template <typename T>

libs/full/collectives/include/hpx/collectives/create_communicator.hpp

@@ -115,6 +115,8 @@ struct hpx::util::extra_data_helper<hpx::collectives::detail::communicator_data>
 namespace hpx::collectives {
 
     ///////////////////////////////////////////////////////////////////////////
+    /// a handle identifying the communication channel to use for a particular
+    /// collective operation
     struct communicator
       : hpx::components::client_base<communicator, detail::communicator_server,
             detail::communicator_data>