[XPath] Range of legal values of a timezone

The XPath language book, section 2.1.2, bullet for implicit-timezone, refers
to ISO 8601: 

"See [ISO 8601] for the range of legal values of a timezone"

However, ISO 8601 doesn't appear to contain any suitable definition we can
appeal to.

We should refer instead to XML Schema Part 2, section 3.2.7.3, which
specifies that timezones are integer multiples of one minute, in the range
-PT14H to +PT14H inclusive.

Michael Kay