rework Node.add_marker parameter list and docstring

src/_pytest/mark/structures.py

@@ -287,7 +287,11 @@ class MarkInfo(object):
     def validate_marks(self, attribute, value):
         for item in value:
             if not isinstance(item, Mark):
-                raise ValueError(item)
+                raise ValueError(
+                    "MarkInfo expects Mark instances, got {!r} ({!r})".format(
+                        item, type(item)
+                    )
+                )
 
     combined = attr.ib(
         repr=False,

src/_pytest/nodes.py

@@ -173,12 +173,10 @@ class Node(object):
         chain.reverse()
         return chain
 
-    def add_marker(self, marker, append=True):
+    def add_marker(self, marker):
         """dynamically add a marker object to the node.
 
-        ``marker`` can be a string or pytest.mark.* instance.
-        ``append=True`` whether to append the marker,
-            if false insert at position 0
+        :type marker: str or pytest.mark.*
         """
         from _pytest.mark import MarkDecorator, MARK_GEN