pmod() sometimes returns negative values

I noticed when constructing examples to compare/contrast fmod(), pmod(), and the new mod() function that pmod() sometimes returns a negative value, despite its being stated to be the "positive" modulus:

[localhost:21000] > select pmod(5,-2);
+-------------+
| pmod(5, -2) |
+-------------+
| -1          |
+-------------+

I see that's the same return value as Hive:

hive> select pmod(5,-2) from t1 limit 1;
Total MapReduce jobs = 1
...
OK
-1
Time taken: 29.269 seconds

This seems to be the origin of the Hive pmod() function, it isn't spec'ed out in detail: https://issues.apache.org/jira/browse/HIVE-656

I thought we should verify that our behavior is working as intended, and if so let's firm up the explanation of the function a big. ("Returns the positive modulus except if second argument negative", or something like that.)