DOC: clarify blend operator terminology in gdal raster blend

[sidhansu10]

This PR clarifies the terminology used by gdal raster blend. The term operator corresponds to blend modes in other GIS and graphics software (for example QGIS or Photoshop), and the note clarifies that the src-over operator corresponds to the commonly known normal blend mode.
Fixes #13433.

[jratike80]

I wonder what are those historical reasons. gdal raster blend was introduced in GDAL 3.12. Maybe they come from Mapserver https://mapserver.org/mapfile/composite.html.

[rouault]

@sidhansu10 Please take into account the alternative phrasing I suggested in #13627 (comment)

[sidhansu10]

@rouault I've updated the note to use the simplified wording suggested earlier and removed the reference to historical reasons.

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