null file explanations from Glynn added

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@34448 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/gis/gisrasterlib.dox

@@ -1643,6 +1643,8 @@ int Dcell() If the map is a floating-point map, read the map using
 
 \section Null_no_data NULL (no data) handling
 
+The <TT>null</TT> file is stored in <TT>cell_misc/name/null file</TT>.
+
 -2^31 (= 0x80000000 = -2147483648) is the null value
 for the CELL type, so you'll never see that value in a map.
 
@@ -1661,6 +1663,18 @@ values are treated as null. This will mean that code which can
 generate NaNs doesn't have to explicitly convert them to the GRASS
 null value.
 
+<b>Presence or absence of <TT>null</TT> file:</b>
+For an integer map, any cells which were null will become zero, but
+any zeroes (cells which were previously either null or zero) will be
+treated as nulls (this is for compatibility with GRASS 4.x, which
+didn't have a <TT>null</TT> file, but typically used zero to indicate a null
+value).
+
+For a floating-point map, any cells which were null will become zero
+(when writing FP data, a null has a zero written to the fcell/<map>
+file, and the corresponding bit is set in the <TT>null</TT>
+file).
+
 \section Color_Functions Color Functions (new and upgraded)
 
 \subsection Upgraded_Colors_structures Upgraded Colors structures