r.accumulate: Update the manual with a new difference map name and LF…

…P citations (#959)

* r.accumulate: Rename accum_diff to flow_accum_diff in the manual

* Add citations for LFP

src/raster/r.accumulate/r.accumulate.html

@@ -1,7 +1,7 @@
 <h2>DESCRIPTION</h2>
 
 <em>r.accumulate</em> calculates weighted flow accumulation, subwatersheds,
-stream networks, and longest flow paths using a flow direction map.
+stream networks, and longest flow paths (Cho 2020) using a flow direction map.
 
 <h2>NOTES</h2>
 
@@ -62,12 +62,12 @@ <h3>Longest flow path calculation</h3>
 
 With <b>longest_flow_path</b> option, the module will create a longest flow
 path vector map for outlet points specified by <b>coordinates</b> and/or
-<b>outlet</b> with <b>outlet_layer</b> option. By default, longest flow paths
-will be created as if there were subwatersheds at the outlets by calculating
-the subaccumulation map. Downstream longest flow paths will never traverse
-through upstream outlets. With <b>-a</b> flag, longest flow paths will be
-created in an accumulated manner resulting in an overlap between downstream and
-upstream longest flow paths.
+<b>outlet</b> with <b>outlet_layer</b> option, using the algorithm by Cho
+(2020). By default, longest flow paths will be created as if there were
+subwatersheds at the outlets by calculating the subaccumulation map. Downstream
+longest flow paths will never traverse through upstream outlets. With <b>-a</b>
+flag, longest flow paths will be created in an accumulated manner resulting in
+an overlap between downstream and upstream longest flow paths.
 
 <p>You can assign unique IDs to longest flow paths using <b>id</b> (with
 <b>coordinates</b>) and/or <b>outlet_id_column</b> (with <b>outlet</b>).
@@ -100,17 +100,17 @@ <h3>Flow accumulation</h3>
 r.colors map=flow_accum_new raster=flow_accum
 
 # check difference between flow_accum and flow_accum_new
-r.mapcalc expression="accum_diff=if(flow_accum-flow_accum_new, flow_accum-flow_accum_new, null())"
+r.mapcalc expression="flow_accum_diff=if(flow_accum-flow_accum_new, flow_accum-flow_accum_new, null())"
 </pre></div>
 
 <img src="r_accumulate_nc_example.png">
 
-<p>For some reason, there are slight differences between the two output maps.
-The yellow and purple cells show the difference raster map (<i>accum_diff</i>).
-The red arrows and numbers represent drainage directions
-(<i>drain_directions</i>) and flow accumulation by <em>r.watershed</em>
-(<i>flow_accum</i>), respectively. Note that some cells close to headwater
-cells are assigned 1 even though they are located downstream of other cells.
+<p>There are slight differences between the two output maps. The yellow and
+purple cells show the difference raster map (<i>flow_accum_diff</i>). The red
+arrows and numbers represent drainage directions (<i>drain_directions</i>) and
+flow accumulation by <em>r.watershed</em> (<i>flow_accum</i>), respectively.
+Note that some cells close to headwater cells are assigned 1 even though they
+are located downstream of other cells.
 
 <p><img src="r_accumulate_r_watershed_nc_example.png">