Update ATTR doc syntax

Author: boriel

docs/attr.md

@@ -2,11 +2,16 @@
 
 ATTR is a library function to be included with the following command:
 
-
 ```
 #include <attr.bas>
 ```
 
+##Description
+`ATTR(row, col)`
+
+Returns the ATTR (color attribute) of the given screen coordinate at the given row and column.
+
+
 ##Sample usage
 
 ```