Extended PyDocStringOwner interface and moved docstring utilities into DocStringUtil

python/psi-api/src/com/jetbrains/python/psi/PyDocStringOwner.java

@@ -3,6 +3,12 @@ package com.jetbrains.python.psi;
 import org.jetbrains.annotations.Nullable;
 
 public interface PyDocStringOwner extends PyElement {
+  @Nullable
+  String getDocStringValue();
+
+  @Nullable
+  StructuredDocString getStructuredDocString();
+
   @Nullable
   PyStringLiteralExpression getDocStringExpression();
 }

python/psi-api/src/com/jetbrains/python/psi/PyFunction.java

@@ -48,9 +48,6 @@ extends
   @Nullable
   PyType getReturnTypeFromDocString();
 
-  @Nullable
-  String getDocStringValue();
-
   /**
    * If the function raises a DeprecationWarning or a PendingDeprecationWarning, returns the explanation text provided for the warning..
    *