Pragma `key_not_null` is not documented

[roe85]

I was porting from ODB 2.4.0 to 2.5.0 where a found a schema change in a QHash<QString, QString>, because nullability has been changed to be the default. Was made aware of the pragma value_not_null which also works for normal containers and I tried out key_not_null by chance - and it worked.

But it is not documented anywhere, I just found it used in the source code.

[boris-kolpackov]

Yes, you are right, it seems to be missing for some reason. Will fix, thanks for the report.