DOC: Add warning about surface brightness in Simple Aperture Photomet…

…ry plugin (#2261)

* DOC: Add warning about surface brightness
in Simple Aperture Photometry plugin.

I am beginning to think the Simple in plugin name no longer applies.

* DOC: Improve verbiage.

Co-authored-by: Camilla Pacifici <camilla.pacifici@gmail.com>

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Co-authored-by: Camilla Pacifici <camilla.pacifici@gmail.com>

docs/imviz/plugins.rst

@@ -202,6 +202,15 @@ an interactively selected region. A typical workflow is as follows:
    in display data unit. Otherwise, it is only informational.
    If this field is not applicable for you, leave it at 0.
    **This field resets every time Data selection changes if auto-population not possible.**
+
+   .. warning::
+
+       If your data is in surface brightness units and pixels on the image
+       have varying sky area, you should first convert your data from
+       surface brightness to flux units before using this plugin.
+       This is because, for performance reasons, the plugin multiplies
+       by the area after the aperture sum is calculated.
+
 7. If you also want photometry result in the unit of counts, you can enter a
    conversion factor in the :guilabel:`Counts conversion factor` field. The value
    must be in the unit of display data unit per counts. This is used to convert linear