fix(rbt): improve naming and add docstrings

Signed-off-by: Matej Focko <mfocko@redhat.com>

rbt.py

@@ -10,12 +10,24 @@ T = TypeVar("T", bound=Comparable)
 
 
 class Colour(enum.IntEnum):
+    """
+    Represents colour of the edge or node.
+    """
     Red = 0
     Black = 1
 
 
-def is_double_black(x: Optional[Node]) -> bool:
-    return x and 2 in Node.differences(x)
+def has_double_black(x: Optional[Node]) -> bool:
+    """
+    Checks for double black child of x.
+
+    Args:
+        x: Node to be checked.
+
+    Returns:
+        `true`, if `x` has a double black node, `false` otherwise.
+    """
+    return x is not None and 2 in Node.differences(x)
 
 
 class RBTree(RankedTree[T]):
@@ -143,7 +155,7 @@ class RBTree(RankedTree[T]):
         if not node and not parent:
             return
 
-        while node != self.root and is_double_black(parent):
+        while node != self.root and has_double_black(parent):
             if node == parent.left:
                 node, parent = self._delete_rebalance_step(
                     node,