Fix a logic error that causes annotations not to be shown for descriptors

This `if not self._datadescriptor` was originally here before type annotations were supported, to prevent showing `descr4 = <_Descriptor ...>` in the docs.

When type annotations were added, the support for appending `:type:` was put in the wrong place.
It should have been outside this if, not within it.

sphinx/ext/autodoc/__init__.py

@@ -1584,8 +1584,7 @@ class AttributeDocumenter(DocstringStripSignatureMixin, ClassLevelDocumenter):
         super().add_directive_header(sig)
         sourcename = self.get_sourcename()
         if not self.options.annotation:
-            if not self._datadescriptor:
-                # obtain annotation for this attribute
+            # obtain type annotation for this attribute
             annotations = getattr(self.parent, '__annotations__', {})
             if annotations and self.objpath[-1] in annotations:
                 objrepr = stringify_typehint(annotations.get(self.objpath[-1]))
@@ -1596,6 +1595,8 @@ class AttributeDocumenter(DocstringStripSignatureMixin, ClassLevelDocumenter):
                     self.add_line('   :type: ' + self.analyzer.annotations[key],
                                   sourcename)
 
+            # data descriptors do not have useful values
+            if not self._datadescriptor:
                 try:
                     objrepr = object_description(self.object)
                     self.add_line('   :value: ' + objrepr, sourcename)

tests/roots/test-ext-autodoc/target/typed_vars.py

@@ -6,11 +6,20 @@ attr2: str
 attr3 = ''  # type: str
 
 
+class _Descriptor:
+    def __init__(self, name):
+        self.__doc__ = "This is {}".format(name)
+    def __get__(self):
+        pass
+
+
 class Class:
     attr1: int = 0
     attr2: int
     attr3 = 0  # type: int
 
+    descr4: int = _Descriptor("descr4")
+
     def __init__(self):
         self.attr4: int = 0     #: attr4
         self.attr5: int         #: attr5

tests/test_autodoc.py

@@ -1518,6 +1518,13 @@ def test_autodoc_typed_instance_variables(app):
         '      attr6',
         '',
         '',
+        '   .. py:attribute:: Class.descr4',
+        '      :module: target.typed_vars',
+        '      :type: int',
+        '',
+        '      This is descr4',
+        '',
+        '',
         '.. py:data:: attr1',
         '   :module: target.typed_vars',
         '   :type: str',