Add a docstring describing UnInlt

Author: rlskoeser

src/undate/date.py

@@ -33,6 +33,13 @@ def days(self) -> int:
 
 @dataclass
 class UnInt:
+    """An uncertain integer intended for use with uncertain durations (:class:`UnDelta`),
+    to convey a range of possible integer values between an upper
+    and lower bound (inclusive). Supports comparison, addition and subtraction,
+    checking if a value is included in the range, and iterating over numbers
+    included in the range.
+    """
+
     lower: int
     upper: int