doxygen cosmetics: document `format: -1`

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@52468 15284696-431f-4ddb-bdfa-cd5b030d7da7

include/gis.h

@@ -321,7 +321,10 @@ enum
 */
 struct Cell_head
 {
-    /*! \brief Max number of bytes per raster data value minus 1 (raster header only) */
+    /*! \brief Max number of bytes per raster data value minus 1 (raster header only)
+
+     Note: -1 for FP raster maps
+    */
     int format;
     /*! \brief Compression mode (raster header only)