Add class docstring to NodeMeta (#11427)

Author: WarrenTheRabbit

src/_pytest/nodes.py

@@ -127,6 +127,20 @@ def _imply_path(
 
 
 class NodeMeta(type):
+    """Metaclass used by :class:`Node` to enforce that direct construction raises
+    :class:`Failed`.
+
+    This behaviour supports the indirection introduced with :meth:`Node.from_parent`,
+    the named constructor to be used instead of direct construction. The design
+    decision to enforce indirection with :class:`NodeMeta` was made as a
+    temporary aid for refactoring the collection tree, which was diagnosed to
+    have :class:`Node` objects whose creational patterns were overly entangled.
+    Once the refactoring is complete, this metaclass can be removed.
+
+    See https://github.com/pytest-dev/pytest/projects/3 for an overview of the
+    progress on detangling the :class:`Node` classes.
+    """
+
     def __call__(self, *k, **kw):
         msg = (
             "Direct construction of {name} has been deprecated, please use {name}.from_parent.\n"