Document r.external issues with r.null (#91)

* Add r.external issues with r.null

raster/r.null/r.null.html

@@ -4,7 +4,7 @@ The function of <em>r.null</em> is to explicitly create the NULL-value
 bitmap file. The intended usage is to update maps that do not have a
 NULL-value bitmap file (i.e. to indicate for each pixel if zero is a valid
 value or is to be considered as NULL, i.e. no data value). The module does
-not work with reclassified maps.
+not work with reclassified or external maps.
 
 <p>
 The design is flexible. Ranges of values can be set to NULL and/or the NULL
@@ -45,6 +45,15 @@ explicitly turned on with <tt>export GRASS_COMPRESS_NULLS=1</tt>.<br>
 Warning: such raster maps can then only be opened with GRASS GIS 7.2.0 or
 later. NULL file compression can be managed with <b>r.null -z</b>.
 
+<h3>External maps</h3>
+From the <a href="r.external.html">r.external</a> documentation: GDAL-linked 
+(<em>r.external</em>) maps do not have or use a NULL bitmap, hence <em>r.null</em> cannot 
+manipulate them directly. Here NULL cells are those whose value matches 
+the value reported by the GDALGetRasterNoDataValue() function. To apply the 
+GDAL-linked the user need to either create a MASK (e.g. with <em>r.mask</em>) and 
+then "apply" it using e.g. <em>r.resample</em>, or use <em>r.mapcalc</em> to create a copy 
+with the appropriate categories changed to NULL (<tt>if()</tt> condition).
+
 <h2>EXAMPLES</h2>
 Set specific values of a classified map to NULL:<br>
 <div class="code"><pre>