Add note about SLD_VERSION (#1069)

Co-authored-by: sethg <sethg@geographika.co.uk>

Author: github-actions[bot]

en/ogc/wms_server.txt

@@ -565,7 +565,9 @@ The following are valid parameters for this GetLegendGraphic request:
   - **SLD_BODY** - (Optional) The body (code) of the SLD, instead of
     specifying a URL (as in the 'SLD' parameter).
 
-  - **SLD_VERSION** - (Optional) The SLD version.
+  - **SLD_VERSION** - (Optional) Version of the SLD specification used when
+    SLD or SLD_BODY is provided. When using WMS VERSION=1.3.0, MapServer
+    requires this parameter, and it must be set to "1.1.0".
 
   - **SCALE** - (Optional) Specify a scale so that only layers that
     fall into that scale will have a legend.