docs(constants): Say what the default-scope sentinel means

It is the default value of every `scope` parameter and appeared in the API
reference as a name with a `...` body. The distinction worth stating is that
it is not the same as passing no scope: tmux resolves the option itself.

Author: tony

src/libtmux/constants.py

@@ -54,8 +54,14 @@ class PaneDirection(enum.Enum):
 
 
 class _DefaultOptionScope:
-    # Sentinel value for default scope
-    ...
+    """Sentinel meaning "whichever scope tmux would use".
+
+    Distinct from passing no scope at all: this is the default value of the
+    ``scope`` parameter, and a call carrying it sends no scope flag. tmux then
+    resolves the option itself, which for a pane option means falling back to
+    the window's and then to the global window options.
+    """
+
 
 
 DEFAULT_OPTION_SCOPE: _DefaultOptionScope = _DefaultOptionScope()