lavfi/avfilter: document AVFilterContext.is_disabled as private

Ideally there should be three parts to the filter context - public,
private to the filter, and private to generic code, but only the first
and the last of these exist currently. Until the second is implemented,
this is better than nothing.

libavfilter/avfilter.h

@@ -518,7 +518,12 @@ struct AVFilterContext {
      */
     double *var_values;
 #endif
-    int is_disabled;                ///< the enabled state from the last expression evaluation
+    /**
+     * MUST NOT be accessed from outside avfilter.
+     *
+     * the enabled state from the last expression evaluation
+     */
+    int is_disabled;
 
     /**
      * For filters which will create hardware frames, sets the device the