Added documentation for geomtransform expressions (#4332)

en/mapfile/style.txt

@@ -174,6 +174,9 @@ GAP [double]
 .. index::
    triple: STYLE; GEOMTRANSFORM; vertices
 
+.. index::
+   triple: STYLE; GEOMTRANSFORM; expression
+
 GEOMTRANSFORM [bbox|end|labelpnt|labelpoly|start|vertices]
     Used to indicate that the current feature will be transformed
     before the actual style is applied. Introduced in version 5.4.
@@ -196,6 +199,29 @@ GEOMTRANSFORM [bbox|end|labelpnt|labelpoly|start|vertices]
       the current feature geometry (the start and end are excluded).
       When used with `ANGLE` AUTO, the marker is oriented by the half
       angle formed by the two adjacent line segments.
+    - *<expression>*: Applies the given expression to the geometry.
+       Supported expressions:
+
+       .. index::
+          triple: STYLE; GEOMTRANSFORM; buffer
+
+       - *(buffer([shape],dist)*
+          Buffer the geometry (`[shape]`) using `dist` pixels as
+          buffer distance.  For polygons, a negative `dist` will
+          produce a setback.
+
+          Example (polygon data set) - draw a two pixel wide line 5
+          pixels inside the boundary of the polygon::
+
+            STYLE
+              OUTLINECOLOR 255 0 0
+              WIDTH 2
+              GEOMTRANSFORM (buffer([shape],-5))
+            END
+
+          .. note::
+
+              Depends on GEOS.
 
 .. index::
    pair: STYLE; INITIALGAP