d.rast.edit: Use raster mask, not uppercase MASK (#5001)

Replaces usage of uppercase MASK by 'raster mask' in the documentation. This clarifies the sentence in terms of what respecting means (original says 'respected...including any edits' which may suggest the opposite). It gives me too many errors to verify the behavior as the comments asks for, but the code uses r.patch, so the mask is likely applied and thus the mask is applied for any writing (unlike r.mapcalc).

scripts/d.rast.edit/d.rast.edit.html

@@ -49,9 +49,10 @@ <h2>NOTES</h2>
 There is no "undo" command nor way to exit edit mode without saving
 changes to the output map.
 <p>
-<!-- MASK behaviour to be verified -->
-Any MASK in place will be respected when writing the new raster map layer,
-including any edits to the MASKed areas.
+<!-- masking behaviour to be verified -->
+A raster mask in place will be respected both when reading the original raster
+map layer and when writing the new raster map layer,
+so any edits to the masked areas will be ignored.
 
 <h3>Geographic Region Concerns</h3>