Update description of 2D array indices

docs/getting_started/pixel_conventions.rst

@@ -1,11 +1,11 @@
 Pixel Coordinate Conventions
 ============================
 
-In Photutils, integer pixel coordinates fall at the center of pixels,
-and they are 0-indexed, matching the Python 0-based indexing. That means
-the first pixel is considered pixel ``0``, but pixel coordinate ``0`` is
-the *center* of that pixel. Hence, the first pixel spans pixel values
-``-0.5`` to ``0.5``.
+In Photutils, integer pixel coordinates are located at the center of
+pixels, and they are 0-indexed, matching the Python 0-based indexing.
+That means the first pixel is considered pixel ``0``, but pixel
+coordinate ``0`` is the *center* of that pixel. Hence, the first pixel
+spans pixel values ``-0.5`` to ``0.5``.
 
 For a 2-dimensional array, ``(x, y) = (0, 0)`` corresponds to
 the *center* of the bottom, leftmost array element. That means
@@ -15,10 +15,10 @@ WCS <https://fits.gsfc.nasa.gov/fits_wcs.html>`_, `ds9`_, and
 `SourceExtractor`_ conventions, in which the center of the bottom,
 leftmost array element is ``(x, y) = (1, 1)``.
 
-Following Python indexing, the ``x`` (column) coordinate corresponds to
-the second (fast) array index and the ``y`` (row) coordinate corresponds
-to the first (slow) index. ``image[y, x]`` gives the value at pixel
-coordinates ``(x, y)``.
+Following Python indexing, two-dimensional arrays are indexed as
+``image[yi, xi]``, with 0 being the first index. The ``xi`` (column)
+index corresponds to the second (fast) array index and the ``yi`` (row)
+index corresponds to the first (slow) index.
 
 .. _SourceExtractor: https://sextractor.readthedocs.io/en/latest/
 .. _ds9: http://ds9.si.edu/