Potential mistake in docs regarding operator parameter in CompoundSelect
Wow, no the docs are definitely wrong. I'll update them, thanks.
I'm using peewee 2.9.1 with Python 3.5.2 on Linux Mint 18 to access a PostgreSQL 9.6.2 database hosted on AWS RDS.

The [documentation for CompoundSelect](http://docs.peewee-orm.com/en/latest/peewee/api.html#CompoundSelect) states that one should use a Node instance for the operator parameter in a compound query. I tried each of SQL('UNION'), SQL('INTERSECTION'), and SQL('EXCEPT'), but kept getting errors from psycopg2 and peewee with the syntax of the formatted SQL.

An example of the formatted SQL is below:

```
>>> print(inds.sql()
```
`('(SELECT "t1"."id", "t1"."recid", "t1"."year", "t1"."papername", "t1"."timeofday", "t1"."newspaper_id" FROM "newspaperyears" AS t1 WHERE ("t1"."year" = %s)) <peewee.SQL object at 0x7f2d702d6518> (SELECT "t3"."id", "t3"."recid", "t3"."year", "t3"."papername", "t3"."timeofday", "t3"."newspaper_id" FROM "newspaperyears" AS t3 INNER JOIN "newspapers" AS t4 ON ("t3"."newspaper_id" = "t4"."id") WHERE ("t3"."year" = %s))', [1952, 1952])`

It seems like either peewee's or psycopg2's SQL generation was directly inserting a reference to the Node instance. Noticing this, I instead simply used 'EXCEPT' instead of SQL('EXCEPT') for the operator parameter, and I was able to get the CompoundSelect query that I wanted.

I checked the commit history, and it didn't seem like there has been anything addressing this type of issue between 2.9.1 and 2.10, but my apologies if I missed something.