Change config key from "single" to "item"

For consistency with the rest of the terminology in the docs/config.
Also, correct the documentation (which previously only covered albums).

beets/config_default.yaml

@@ -29,7 +29,7 @@ import:
     search_ids: []
     duplicate_keys:
         album: albumartist album
-        single: artist title
+        item: artist title
     duplicate_action: ask
     bell: no
     set_fields: {}

beets/importer.py

@@ -937,7 +937,7 @@ def find_duplicates(self, lib):
         # Query for existing items using the same metadata. We use a
         # temporary `Item` object to generate any computed fields.
         tmp_item = library.Item(lib, **info)
-        keys = config['import']['duplicate_keys']['single'].as_str_seq()
+        keys = config['import']['duplicate_keys']['item'].as_str_seq()
         dup_query = library.Album.all_fields_query({
             key: tmp_item.get(key)
             for key in keys

docs/reference/config.rst

@@ -675,10 +675,16 @@ Default: ``yes``.
 duplicate_keys
 ~~~~~~~~~~~~~~
 
-The fields used to find duplicates in import task.
-If several albums have the same value for each key, they will be considered duplicates.
+The fields used to find duplicates when importing.
+There are two sub-values here: ``album`` and ``item``.
+Each one is a list of field names; if an existing object (album or item) in
+the library matches the new object on all of these fields, the importer will
+consider it a duplicate.
 
-Default: ``albumartist album``
+Default::
+
+    album: albumartist album
+    item: artist title
 
 .. _duplicate_action:
 

test/test_importer.py

@@ -1359,7 +1359,7 @@ def setUp(self):
         self.importer = self.create_importer()
         config['import']['autotag'] = True
         config['import']['singletons'] = True
-        config['import']['duplicate_keys']['single'] = 'artist title'
+        config['import']['duplicate_keys']['item'] = 'artist title'
 
     def tearDown(self):
         self.teardown_beets()
@@ -1397,7 +1397,7 @@ def test_skip_duplicate(self):
         self.assertEqual(item.mb_trackid, 'old trackid')
 
     def test_keep_when_extra_key_is_different(self):
-        config['import']['duplicate_keys']['single'] = 'artist title flex'
+        config['import']['duplicate_keys']['item'] = 'artist title flex'
         item = self.lib.items().get()
         item.flex = 'different'
         item.store()