[docs] Please document sqlite3 return values

[Bjartur Thorlacius]

Dear Python 3 documentation team.

The documentation for Python 3.7.3rc1 and 3.6.8 describes the 
sqlite3.Cursor.fetchone as follows:

|fetchone|()¶ 
<https://docs.python.org/3/library/sqlite3.html#sqlite3.Cursor.fetchone>

    Fetches the next row of a query result set, returning a single
    sequence, or None when no more data is available.

However, when I use the Python 3.7 from the public Docker image 
2afe3d389ba0, fetchone seems to return (0,) when no row matches the 
query. Confusingly, fetchmany and fetchall are only documented as 
returnig "a list". A more specific type declaration or description of 
the hierarchical return type would be appreciated.

Best regards,
Bjartur Thorlacius

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