Clarify the get_cumulative_mass docs

Author: hyanwong

python/tskit/intervals.py

@@ -247,9 +247,10 @@ def get_cumulative_mass(self, x=None):
         given point for a list of positions along the map. This is equal to
         the integral of the rate from 0 to the point.
 
-        .. note:: If the RateMap is a "genetic map" (that is, it measures the
-            rate of recombination along the genome), this function will return
-            the "genetic distance" along the chromosome, e.g. in Morgans
+        .. note:: If the RateMap is a "genetic map" measuring the
+            rate of recombination along a chromosome, this function will return
+            the "genetic distance" (i.e. the map distance) for the provided
+            physical positions e.g. in Morgans.
 
         :param numpy.ndarray x: The positions for which to return values. If None,
             get the cumulative mass at each :attr:`~.RateMap.position` used
@@ -527,7 +528,8 @@ def read_hapmap(
             physical position in the file is marked as unknown. Otherwise, act
             as if an additional row, specifying physical position 0 and genetic
             position 0, exists at the start of the file.
-        :return: A RateMap object.
+        :return: A RateMap object, whose :meth:`get_cumulative_mass` method can
+            be used to translate physical positions into positions in the genetic map.
         :rtype: RateMap
         """
         # fmt: on