Document possible Sqlite datetime issues.

This may also be considered a bug instead.

```
SQLite version 3.7.17 2013-05-20 00:56:22
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table tbl1(dt datetime);
sqlite> insert into tbl1 (dt) VALUES ('2013-05-20 00:56');
sqlite> insert into tbl1 (dt) VALUES ('foo');
sqlite> select * from tbl1;
2013-05-20 00:56
foo
sqlite> select datetime(dt) from tbl1;
2013-05-20 00:56:00

sqlite>
```

Discovered via https://github.com/owncloud/core/issues/6327
Refs https://github.com/owncloud/core/commit/f8088ecd29af75072f96a3841709faf72c6e8fa1