r.watershed: Use raster mask, not MASK, simplify doc (#4998)

* Replaces uppercase MASK by whatever is more appropriate in the given context.
* Simplifies the sentences about mask and nulls cells. They are the same, ignored, and reduce computational requirements. The original text was repetetive. The new text is trying to be more succinct.
* The heading mentions no data, but not NULLs while text NULLs and not no data. The new text is trying to address that.
* An example which is directly manipulating MASK is left as is.

raster/r.watershed/front/r.watershed.html

@@ -316,17 +316,15 @@ <h3>Basin threshold</h3>
 stream segment is determined by the distance between the tributaries
 flowing into it.
 
-<h3>MASK and no data</h3>
+<h3>Masking and no data</h3>
 
 
 The <em>r.watershed</em> program does not require the user to have the
 current geographic region filled with elevation values.  Areas without
-elevation data (masked or NULL cells) are ignored. It is NOT necessary
-to create a raster map (or raster reclassification)
-named <code>MASK</code> for NULL cells.  Areas without elevation data will
-be treated as if they are off the edge of the region. Such areas will
-reduce the memory necessary to run the program.  Masking out
-unimportant areas can significantly reduce processing time if the
+elevation data (masked or NULL cells, i.e., no data) are ignored and
+treated as if they were outside the region. Masking out
+unimportant areas can significantly reduce processing time and memory
+requirements if the
 watersheds of interest occupy a small percentage of the overall area.
 
 <p>
@@ -357,7 +355,7 @@ <h3>Further processing of output layers</h3>
 module, a number of approaches may be considered.
 
 <ol>
-<li>Use a resample of the basins catchment raster map as a MASK.<br>
+<li>Use a resample of the basins catchment raster map as a raster mask.<br>
   The equivalent vector map method is similar
   using <em><a href="v.select.html">v.select</a></em> or
   <em><a href="v.overlay.html">v.overlay</a></em>.</li>
@@ -386,7 +384,7 @@ <h3>Further processing of output layers</h3>
 with
 <em><a href="r.thin.html">r.thin</a></em>. Each stream segment in the
 vector map will have the value of the associated basin. To isolate
-subbasins and streams for a larger basin, a MASK for the larger basin
+subbasins and streams for a larger basin, a mask for the larger basin
 can be created with
 <em><a href="r.water.outlet.html">r.water.outlet</a></em>. The stream
 segments output serves as a guide where to place the outlet point used