update text

Author: jeanPerier

flang/docs/Extensions.md

@@ -161,8 +161,9 @@ end
 * `$` and `@` as legal characters in names
 * Initialization in type declaration statements using `/values/`
 * Saved variables without explicit or default initializers are zero initialized,
-  except for implicitly saved scalar variables from the main program that are
-  kept on the stack by flang and are not zero initialized. It is not advised to
+  except for scalar variables from the main program that are not explicitly
+  initialized or marked with an explicit SAVE attribute (these variables may be
+  placed on the stack by flang and not zero initialized). It is not advised to
   rely on this extension in new code.
 * In a saved entity of a type with a default initializer, components without default
   values are zero initialized.