r.mapcalc: Improve documentation to clarify LHS/RHS issues. Fixes https://trac.osgeo.org/grass/ticket/2986 and https://trac.osgeo.org/grass/ticket/2770 (backport from trunk https://trac.osgeo.org/grass/changeset/68268).

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@68269 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.mapcalc/r.mapcalc.html

@@ -680,7 +680,21 @@ X (map) values supplied and y (newmap) values returned:
 
 <h2>KNOWN ISSUES</h2>
 
-Continuation lines must end with a <tt>\</tt> and have <em>no</em> trailing
+<p>The <em>result</em> variable on the left hand side of the equation should not
+appear in the <em>expression</em> on the right hand side.
+<div class="code"><pre>
+<s>mymap = if( mymap > 0, mymap, 0)</s>
+</pre></div>
+<p>Any maps generated by an r.mapcalc command only exist after the entire
+command has completed. All maps are generated concurrently, row-by-row
+(i.e. there is an implicit "for row in rows {...}" around the entire expression).
+Thus #, @ and [ ] operators can not be used on a map generated within same
+r.mapcalc command run.
+<div class="code"><pre>
+newmap = oldmap * 3.14
+<s>othermap = newmap[-1, 0] / newmap[1, 0]</s>
+</pre></div>
+<p>Continuation lines must end with a <tt>\</tt> and have <em>no</em> trailing
 white space (blanks or tabs). If the user does leave white space at the end of
 continuation lines, the error messages produced by <em>r.mapcalc</em> will
 be meaningless and the equation will not work as the user intended.

raster/r.mapcalc/r3.mapcalc.html

@@ -552,7 +552,14 @@ X (map) values supplied and y (newmap) values returned:
 
 <h2>KNOWN ISSUES</h2>
 
-Continuation lines must end with a <tt>\</tt> and have <em>no</em> trailing
+<p>The <em>result</em> variable on the left hand side of the equation should not
+appear in the <em>expression</em> on the right hand side.
+<p>Any maps generated by an r3.mapcalc command only exist after the entire
+command has completed. All maps are generated concurrently, row-by-row
+(i.e. there is an implicit "for row in rows {...}" around the entire expression).
+Thus #, @ and [ ] operators can not be used on a map generated within same
+r3.mapcalc command run.
+<p>Continuation lines must end with a <tt>\</tt> and have <em>no</em> trailing
 white space (blanks or tabs). If the user does leave white space at the end of
 continuation lines, the error messages produced by <em>r3.mapcalc</em> will
 be meaningless and the equation will not work as the user intended.