pytester: add docstrings for Testdir.copy_example

changelog/5669.doc.rst

@@ -0,0 +1 @@
+Add docstring for ``Testdir.copy_example``.

src/_pytest/pytester.py

@@ -630,6 +630,12 @@ class Testdir:
         return p
 
     def copy_example(self, name=None):
+        """Copy file from project's directory into the testdir.
+
+        :param str name: The name of the file for copy.
+        :return: self.tmpdir
+
+        """
         import warnings
         from _pytest.warning_types import PYTESTER_COPY_EXAMPLE