r.topidx: Update the manual with info about unsupported lat/long loca…

…tions (#3488)

raster/r.topidx/r.topidx.html

@@ -1,24 +1,33 @@
 <h2>DESCRIPTION</h2>
 
-<em>r.topidx</em> creates topographic index (wetness index), ln(a/tan(beta)), map from
-elevation map
-<p>where
+<em>r.topidx</em> creates topographic index (wetness index), ln(a/tan(beta)),
+map from elevation map where
 <dl>
-<dd>a: the area of the hillslope per unit contour length that drains through any point,
-<p><dd>tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal).
+<dt>a</dt>
+  <dd>the area of the hillslope per unit contour length that drains through any
+      point,</dd>
+<dt>tan(beta)</dt>
+  <dd>the local surface topographic slope (delta vertical) / (delta
+      horizontal).</dd>
 </dl>
-<p>Input maps may have NULL values.  For example, if you have a MASK for
-a watershed (basin map from <em>r.water.outlet</em>), the
-following command will create a masked elevation map (belev):
+<p>Input maps may have NULL values.  For example, if you have a MASK for a
+watershed (basin map from <em>r.water.outlet</em>), the following command will
+create a masked elevation map (belev):
 <div class="code"><pre>
 r.mapcalc "belev = if(isnull(basin), basin, elev)"
 </pre></div>
 <p>
 <em>r.stats -Anc</em> prints out averaged statistics for topographic index.
 
+<p>Unprojected lat/long locations are not supported. If data is not projected
+in a lat/long location, please create a new location in a projected coordinate
+system and reproject the data into the new projected location. Please run
+<em>r.topidx</em> from that location.
+
 <h2>EXAMPLE</h2>
 
-Running the command below will generate the topographic index map shown in the figure:
+Running the command below will generate the topographic index map shown in the
+figure:
 
 <div class="code"><pre>
 g.region raster=elevation -p