db.select: update man page to indicate that SQL statements have to be one complete query per line

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@71389 15284696-431f-4ddb-bdfa-cd5b030d7da7

db/db.select/db.select.html

@@ -2,7 +2,8 @@
 
 <em>db.select</em> prints result of selection from database based on
 SQL statement read from input file or from standard input to standard
-output.
+output. Each individual query has to be written on one single line and 
+different queries have to be written on separate lines.
 
 <h2>NOTE</h2>