context DOC removed internal flag comment

Because no internal flags were found in the code.
diff --git a/src/context.h b/src/context.h
index ac0dfd4..eba0fa2 100644
--- a/src/context.h
+++ b/src/context.h
@@ -161,8 +161,6 @@
  *
  * Options to change context behavior.
  *
- * Note that the flags 0xFF00 are reserved for internal use.
- *
  * @{
  */