Merge pull request #9 from xapian/james/explicitly-year-ranges-are-al…

…l-digits-only

Be clearer that year ranges can't use 2 digit years.

howtos/range_queries.rst

@@ -195,7 +195,7 @@ The :xapian-class:`DateValueRangeProcessor` is working on value slot 2, with an
 1860 and going forward as far 1959). The second parameter is whether
 it should prefer US style dates or not; since we're looking at US
 states, we've gone for US dates. The :xapian-class:`NumberValueRangeProcessor`
-is as we saw before.
+is as we saw before, which means that it can't cope with two digit years.
 
 This enables us to search for any state that talks about the Spanish
 in its description:
@@ -276,6 +276,11 @@ Or all that joined the union in the 1780s and have a population now over 10 mill
 With a little more work, we could support ranges such as '..5m' to
 mean up to 5 million, or '..750k' for up to 750 thousand.
 
+Similarly, it would be possible to use the same approach to create a custom
+:xapian-class:`ValueRangeProcessor` that could restrict to a range of years, and
+cope with two digit years, as our :xapian-class:`DateValueRangeProcessor` did for
+full dates.
+
 Performance limitations
 -----------------------