glsl: update comment on offset field

The old comment was for the location not the offset, we now use
the field for block members so mention that also.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/compiler/glsl/ir.h

@@ -866,7 +866,7 @@ public:
       unsigned stream;
 
       /**
-       * Location an atomic counter is stored at.
+       * Atomic or block member offset.
        */
       unsigned offset;