[TableGen][Docs] Fix productionlists for assert and dump (llvm#123739)

These were referring to nonexistent grammar tokens instead of `Value`.

Author: jayfoad

llvm/docs/TableGen/ProgRef.rst

@@ -1316,8 +1316,9 @@ output. It is intended for debugging purpose.
   instantiation point of the containing record.
 
 .. productionlist::
-   Dump: "dump"  `string` ";"
+   Dump: "dump" `Value` ";"
 
+The :token:`Value` is an arbitrary string expression.
 For example, it can be used in combination with `!repr` to investigate
 the values passed to a multiclass:
 
@@ -1364,11 +1365,12 @@ The ``assert`` statement checks a boolean condition to be sure that it is true
 and prints an error message if it is not.
 
 .. productionlist::
-   Assert: "assert" `condition` "," `message` ";"
+   Assert: "assert" `Value` "," `Value` ";"
 
-If the boolean condition is true, the statement does nothing. If the
-condition is false, it prints a nonfatal error message. The **message**, which
-can be an arbitrary string expression, is included in the error message as a
+The first :token:`Value` is a boolean condition. If it is true, the
+statement does nothing. If the condition is false, it prints a nonfatal
+error message. The second :token:`Value` is a message, which can be an
+arbitrary string expression. It is included in the error message as a
 note. The exact behavior of the ``assert`` statement depends on its
 placement.