Clarify Symbol as per mmatera

Update builtin/atomic/symbols.py according to current standards.

Author: rocky

mathics/core/symbols.py

@@ -332,13 +332,17 @@ def replace_slots(self, slots, evaluation) -> "Atom":
 
 
 class Symbol(Atom, NumericOperators, EvalMixin):
-    """
-    A Symbol is a kind of Atom that acts as a symbolic variable.
+    """A Symbol is a kind of Atom that acts as a symbolic variable.
 
     All Symbols have a name that can be converted to string.
 
-    A Variable Symbol is a ``Symbol``` that is associated with a ``Definition``
-    that determines its evaluation value.
+    A Variable Symbol is a ``Symbol``` that is associated with a
+    ``Definition`` that has an ``OwnValue`` that determines its
+    evaluation value.
+
+    A Function Symbol, like a Variable Symbol, is a ``Symbol`` that is
+    also associated with a ``Definition``. But it has a ``DownValue``
+    that is used in its evaluation.
 
     We also have Symbols which in contrast to Variables Symbols have
     a constant value that cannot change. System`True and System`False