docs: adding explanation for color in set_facecolor

Author: Borda

lib/matplotlib/axes/_base.py

@@ -1499,7 +1499,7 @@ def set_facecolor(self, color):
 
         Parameters
         ----------
-        color : color
+        color : color defined by string name, char abbreviation or hex, see :doc:`gallery/color/named_colors`
         """
         self._facecolor = color
         self.stale = True