Fix up timetz input so that a date is required only when the specified

Fix up timetz input so that a date is required only when the specified
timezone actually has a daylight-savings rule.  This avoids breaking
cases that used to work because they went through the DecodePosixTimezone
code path.  Per contrib regression failures (mea culpa for not running
those yesterday...).  Also document the already-applied change to allow
GMT offsets up to 14 hours.