doc: images should be not wider then 100% of the parent element

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@61949 15284696-431f-4ddb-bdfa-cd5b030d7da7

man/grassdocs.css

@@ -34,6 +34,10 @@ body{
     border-color: #DDDDDD;
 }
 
+img {
+    max-width: 100%;
+}
+
 hr.header {
     height: 3px;
     color: gray;