Document raster PROCESSING GAMMA option

Refs MapServer/MapServer#7069

Author: rouault

en/input/raster.txt

@@ -468,6 +468,22 @@ are supported in MapServer 4.0 and newer.
 
        PROCESSING "EXTENT_PRIORITY=WORLD"
 
+.. index::
+   triple: LAYER; PROCESSING; GAMMA
+
+**GAMMA=<numeric_value>**
+    This directive (MapServer 8.4+) instructs the GDAL reader to apply a
+    `gamma correction <https://en.wikipedia.org/wiki/Gamma_correction>`__ step.
+    The default value is 1.0 (no gamma correction).
+    It is performed after classification, if any.
+    It does not apply to raw rendering.
+
+    Example:
+
+    ::
+
+       PROCESSING "GAMMA=0.75"
+
 .. index::
    triple: LAYER; PROCESSING; LOAD_FULL_RES_IMAGE
 

en/mapfile/layer.txt

@@ -1383,6 +1383,9 @@ PROCESSING [string]
       that appear between adjacent polygons. A value of 0.5 is usually good
       enough.
 
+      Note that starting with MapServer 8.4, a GAMMA processing option is
+      also available for rasters, but its default value is 1.0.
+
       .. code-block:: mapfile
 
           PROCESSING "GAMMA=0.5"