DOC: show disallowed eval syntax
closes #4898

@jreback @jtratner speak now or forever `git revert` your peace.
 I'd say that this is not an exhaustive list.
 e.g., "We've made an effort to keep this syntax simple and clean. As of now, these syntax elements are not supported and support is not planned for the future (not an exhaustive list)."
 > I'd say that this is not an exhaustive list.

I did look at the set of disallowed nodes for the `pandas` parser, so I would say it is (unless I missed something), as far as user-facing syntactic categories go.
 great - then never mind :)
 Here are the disallowed expression nodes:

```
frozenset(['Set', 'SetComp', 'Repr', 'Yield', 'Call', 'DictComp', 'Dict', 'IfExp', 'GeneratorExp', 'Lambda'])
```
 So you're right, I missed yield and generators
 I think `Repr` is for interactive stuff....let me check
 maybe have allowed syntax just to make it obvious?
 I mean in addition to disalllowed
 let me see how long that list is ....
 list is pretty long .. i'll just add a smaller list encompassing a bunch of ops per bullet point
 @jreback @jtratner merge?
 I would switch order, put allowed first, maybe call it `Allowed syntax?` - or if you can think of a better name
 okay ... what do u think?
 good
