Added docstring support to @agent

Fixes issue JuliaDynamics#715. With thanks to Michael Hatherly!

src/core/agents.jl

@@ -192,13 +192,18 @@ macro agent(new_name, base_type, extra_fields)
                     $(base_fields...)
                     $(additional_fields...)
                 end
+                #@doc @doc(name) $name # $docstring $name
             end
-            # @show expr # uncomment this to see that the final expression looks as desired
             # It is important to evaluate the macro in the module that it was called at
             Core.eval($(__module__), expr)
         end
+        # allow attaching docstrings to the new struct, issue #715
+        Core.@__doc__($(esc(Docs.namify(new_name))))
+        nothing
     end
 end
+
+
 # TODO: I do not know how to merge these two macros to remove code duplication.
 # There should be away that only the 4-argument version is used
 # and the 3-argument version just passes `AbstractAgent` to the 4-argument.
@@ -232,8 +237,11 @@ macro agent(new_name, base_type, super_type, extra_fields)
             end
             # @show expr # uncomment this to see that the final expression looks as desired
             # It is important to evaluate the macro in the module that it was called at
-            Core.eval($(__module__), expr)
+            Core.eval($(__module__), Core.@__doc__(expr))
         end
+        # allow attaching docstrings to the new struct, issue #715
+        Core.@__doc__($(esc(Docs.namify(new_name))))
+        nothing
     end
 end