update Termination trait docs

Author: ZHANGWENTAI

library/std/src/process.rs

@@ -2030,6 +2030,10 @@ pub fn id() -> u32 {
 ///
 /// The default implementations are returning `libc::EXIT_SUCCESS` to indicate
 /// a successful execution. In case of a failure, `libc::EXIT_FAILURE` is returned.
+///
+/// For the reason that different runtimes have diffrent specificatons on the
+/// return value of the `main` function, this trait is likely to be available 
+/// only on standard library's runtime for type convenience.
 #[cfg_attr(not(test), lang = "termination")]
 #[unstable(feature = "termination_trait_lib", issue = "43301")]
 #[rustc_on_unimplemented(