message: Document missing value productions
diff: @@ -42,7 +42,9 @@ _core.term_ ::=\
  _core.value_ ::=\
  &emsp;| `stuck(`_core.head_`,` _core.elim_*`)`\
  -&emsp;| TODO
  +&emsp;| `universe`\
  +&emsp;| `fun/type(`_core.value_`,` _core.closure_`)`\
  +&emsp;| `fun/intro(`_core.closure_`)`
  _core.head_ ::=\
  &emsp;| `bound-var(`_core.index_`)`\
  