[common] use 'parent' value as 'parent-metadata' default (#8525 #8604)

enable 'parent-metadata' by default for
'chevereto' & 'imagehost' (album/gallery) extractors

docs/configuration.rst

@@ -231,10 +231,13 @@ Type
     * ``bool``
     * ``string``
 Default
+    ``true``
+        ``[chevereto]`` |
+        ``[imagehost]``
     ``false``
+        otherwise
 Description
-    If ``true``, overwrite any metadata provided by a child extractor
-    with its parent's.
+    Forward a parent's metadata to its child extractors.
 
     | If this is a ``string``, add a parent's metadata to its children's
       to a field named after said string.

docs/gallery-dl.conf

@@ -1007,6 +1007,11 @@
             "videos" : true
         },
 
+        "chevereto":
+        {
+            "parent-metadata": true
+        },
+
         "Danbooru":
         {
             "sleep-request": "0.5-1.5",
@@ -1076,6 +1081,11 @@
             "referer": false
         },
 
+        "imagehost":
+        {
+            "parent-metadata": true
+        },
+
         "mastodon":
         {
             "access-token": null,