Improve doc about 0000-00-00 date/datetime when loc is not UTC

[dolmen]

In #783 some documentation has been added about the driver behaviour for 0000-00-00 when parseTime is true.

But the zero value of time.Time is in location UTC. What happens when the loc option is set to another timezone than UTC?
The documentation should clarify this.

Cc: @methane

[methane]

Zero value is well defined world. It's very clear.
And it's not UTC, it's loc is nil.