documentation extension request - order with function over aggregated functions

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.5/queries-order.html
Description:

In https://www.postgresql.org/docs/9.5/queries-order.html it is said that
the following is intentionally not allowed:

```
SELECT a + b AS sum, c FROM table1 ORDER BY sum + c;          -- wrong
```

It took me a while to figure out how to do it properly so I propose to add
the following (or similar) info:

---
If you need to sort by this kind of expression, you will have to retrieve an
extra column with the required computation and use it as a sorting key:

```
SELECT a + b AS sum, c FROM table1, a + b + c AS my sorting_key  ORDER BY
my_sorting_key;          -- correct
```