provide essential hint to usage

asterisk/asterisk.txt

@@ -710,17 +710,20 @@ limit               = 10
 ;;  func_odbc.conf -- Asterisk ODBC dialplan function configuration
 ;;
 
+;   Set(<variable_name>=${ODBC_SQL(SELECT [...])})
+;   Set(ODBC_SQL(UPDATE [...]))
 [SQL]
 prefix              = ODBC
 dsn                 = asterisk-sqlite
 readsql             = ${ARG1}
+writesql            = ${ARG1}
 
+;   Set(<variable_name>=${ODBC_MAP(<key>)})
+;   Set(ODBC_MAP(<key>)=<value>)
 [MAP]
 prefix              = ODBC
 dsn                 = asterisk-sqlite
-                    ; Set(<variable_name>=${ODBC_MAP(<key>)})
 readsql             = SELECT val FROM map WHERE key='${SQL_ESC(${ARG1})}'
-                    ; Set(ODBC_MAP(<key>)=<value>)
 writesql            = UPDATE map SET val='${SQL_ESC(${VAL1})}' WHERE key='${SQL_ESC(${ARG1})}'
 escapecommas        = no