Add comments RE 'default_value_class' and update the BaseStreamBlock …

…class instead of just StreamBlock (it's a sensible default for both)

wagtail/blocks/stream_block.py

@@ -75,6 +75,9 @@ def as_json_data(self):
 
 
 class BaseStreamBlock(Block):
+ # This is set to `StreamValue` a little further down. The idea is that
+ # developers can override this on custom subclasses to change the default
+ # `value_class` for all instances
  default_value_class = None
 
  def __init__(self, local_blocks=None, search_index=True, **kwargs):
@@ -769,6 +772,11 @@ def __reduce__(self):
  )
 
 
+# Now that StreamValue is defined, make it the default `value_class`
+# for all `BaseStreamBlock` subclasses
+BaseStreamBlock.default_value_class = StreamValue
+
+
 class StreamBlockAdapter(Adapter):
  js_constructor = "wagtail.blocks.StreamBlock"
 
@@ -813,7 +821,4 @@ def media(self):
  ]
  )
 
-
-StreamBlock.default_value_class = StreamValue
-
 register(StreamBlockAdapter(), StreamBlock)