t.rast.algebra: improve neighborhood modifier documentation (#2836)

* document neighborhood modifier

Fix #1065

---------

Co-authored-by: Veronica Andreo <veroandreo@gmail.com>

temporal/t.rast.algebra/t.rast.algebra.html

@@ -445,7 +445,8 @@ <h3>Temporal neighbourhood modifier</h3>
 The neighbourhood modifier of <em>r.mapcalc</em> is extended for the temporal 
 raster algebra with the temporal dimension. The format is strds[t,r,c], 
 where t is the temporal offset, r is the row offset and c is the column 
-offset.
+offset. A single neighborhood modifier is interpreted as temporal offset [t],
+while two neighborhood modifiers are interpreted as row and column offsets [r,c].
 
 <pre class="code">
 strds[2]