Improve _compute_slope docstring

Add more infomration on the purpose of the
_compute_slope method.

Author: elevans

src/imagej/dims.py

@@ -301,8 +301,14 @@ def _get_linear_axis(axis_type: "jc.AxisType", scale_array: np.ndarray, values):
 
 
 def _compute_slope(axis):
-    """
-    Compute slope (i.e. scale) of an axis with all points.
+    """Compute slope (i.e. scale) of an axis.
+
+    Each axis will have an associated coodinates array (1D) which
+    can be linear or non-linear (xarray will use an index as the
+    coordinate array if none is given). This method computes the
+    slope of a given coordinates array using all points in the
+    array. The calculated slope is then used as the 'scale'
+    parameter for creating an ImageJ2 DefaultLinenarAxis.
 
     :param axis: List like 1D array with indexing.
     :return: Slope of the axis.