Update r.mapcalc.txt

Better description of r.mapcalc

Author: pcav

python/plugins/processing/algs/grass7/description/r.mapcalc.txt

@@ -2,7 +2,7 @@ r.mapcalc
 Raster map calculator. 
 Raster (r.*)
 ParameterMultipleInput|maps|Raster maps used in the calculator|3|True
-ParameterString|expression|Expression to evaluate. The raster names used in expression should be the same than in QGIS|None|True|True
+ParameterString|expression|Expression to evaluate. Syntax e.g. `raster_out=raster1+raster2`. Use original filenames, without extension|None|True|True
 ParameterFile|file|File containing expression(s) to evaluate (same rule for raster names than above)|True
 ParameterString|seed|Integer seed for rand() function|None|False|True
 *ParameterBoolean|-s|Generate random seed (result is non-deterministic)|False