DOC: Improve NonUniformImage docs (matplotlib#29370)

* DOC: Improve NonUniformImage docs

* Update lib/matplotlib/image.py

Co-authored-by: David Stansby <dstansby@gmail.com>

---------

Co-authored-by: David Stansby <dstansby@gmail.com>

Author: timhoffm

galleries/examples/images_contours_and_fields/image_nonuniform.py

@@ -3,9 +3,12 @@
 Image nonuniform
 ================
 
-This illustrates the NonUniformImage class.  It is not
-available via an Axes method, but it is easily added to an
-Axes instance as shown here.
+`.NonUniformImage` is a generalized image with pixels on a rectilinear grid,
+i.e. it allows rows and columns with individual heights / widths.
+
+There is no high-level plotting method on `~.axes.Axes` or `.pyplot` to
+create a NonUniformImage. Instead, you have to instantiate the image
+explicitly add it to the Axes using `.Axes.add_image`.
 """
 
 import matplotlib.pyplot as plt

lib/matplotlib/image.py

@@ -826,7 +826,7 @@ def get_filterrad(self):
 
 class AxesImage(_ImageBase):
     """
-    An image attached to an Axes.
+    An image with pixels on a regular grid, attached to an Axes.
 
     Parameters
     ----------
@@ -1013,6 +1013,14 @@ def get_cursor_data(self, event):
 
 
 class NonUniformImage(AxesImage):
+    """
+    An image with pixels on a rectilinear grid.
+
+    In contrast to `.AxesImage`, where pixels are on a regular grid,
+    NonUniformImage allows rows and columns with individual heights / widths.
+
+    See also :doc:`/gallery/images_contours_and_fields/image_nonuniform`.
+    """
 
     def __init__(self, ax, *, interpolation='nearest', **kwargs):
         """