docs: update friction map info for r.walk.html (OSGeo#3279)

raster/r.walk/r.walk.html

@@ -64,6 +64,9 @@ <h2>NOTES</h2>
 
 <p>The <b>friction</b> cost parameter represents a time penalty in seconds
 of additional walking time to cross 1 meter distance.
+Friction cost can be any floating point value &ge; 0. 
+A friction map is a required parameter; if no friction costs are desired,
+a friction map should be a raster in which all cells have a value of 0.
 <p>The <b>lambda</b> parameter is a dimensionless scaling factor of the friction cost:
 
 <div class="code"><pre>