Document ordering expectations for raw SQL

Queries of the form

```C#
...
.Take()
.Include().ThenInclude()
```

are translated the following way:

```SQL
SELECT ...
FROM (
SELECT ...
FROM (...
) as l
LIMIT... -- Take()
) as t
JOIN ... -- Include() entity
JOIN ... -- ThenInclude() entity
ORDER BY t... l... -- the extra ORDER BY
```
An additional `ORDER BY` overrides any ordering of the most inner FROM (or natural ordering) if it's a raw query.

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EF Core version: both 3.1 and 5
Database provider: (tested on PG)
Target framework: .NET Core 3.1