Explain that SELECT results are unsorted

[warpech]

As @miyconst writes in https://github.com/Starcounter/Blending/pull/320#discussion_r230687165:

The SQL SELECT result is never guaranteed to be in any specific order without ORDER BY

Can we please add information about this to the docs?

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I understand that it is by SQL standard (and also seen in Oracle, SQLite, etc), but might be unexpected, compared to some other SQL implementations:

When you query data from a table, PostgreSQL returns the rows in the order that they were inserted into the table.

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