bpo-36523: Add docstring to io.IOBase.writelines (GH-12683)

(cherry picked from commit ab86521a9d9999731e39bd9056420bb7774fd144)
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 0c0cb84..afbd48e 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -546,6 +546,11 @@
         return lines
 
     def writelines(self, lines):
+        """Write a list of lines to the stream.
+
+        Line separators are not added, so it is usual for each of the lines
+        provided to have a line separator at the end.
+        """
         self._checkClosed()
         for line in lines:
             self.write(line)