[flang][doc] refine zero initialization extension documentation (#121956)

Following-up on [comments
](#62432 (comment))
in the issue that motivated this extension.

Author: jeanPerier

flang/docs/Extensions.md

@@ -160,7 +160,11 @@ end
 * `<>` as synonym for `.NE.` and `/=`
 * `$` and `@` as legal characters in names
 * Initialization in type declaration statements using `/values/`
-* Saved variables without explicit or default initializers are zero initialized.
+* Saved variables without explicit or default initializers are zero initialized,
+  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.
 * Kind specification with `*`, e.g. `REAL*4`