Clarify documentation of Cursor get* methods.

Make clear in the Javadoc comments of the `Cursor` get* methods that
implementations thereof can have implementation-defined behavior. In some cases,
these changes actually correct the documentation. For example, in the case of
`getShort` and the `SQLiteCursor` implementation thereof, non-numeric data is
*not* converted to a `short` via Short#valueOf or even in a functionally-
equivalent manner.

Change-Id: Ib2f81811a603680b52fc482eb9c0f3195447566f