Update r.walk.html

Add text to clarify friction map requirements and values.

raster/r.walk/r.walk.html

@@ -63,7 +63,7 @@ <h2>NOTES</h2>
 standard conditions.
 
 <p>The <b>friction</b> cost parameter represents a time penalty in seconds
-of additional walking time to cross 1 meter distance.
+of additional walking time to cross 1 meter distance. Friction cost can be any floating point value but cannot be negative (ie, friction≥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:
 
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