Update the library stabilization guide to refer to the new placeholder system

Author: est31

src/feature-lifecycle/stabilization.md

@@ -64,13 +64,13 @@ Library items are marked unstable via the `#[unstable]` attribute, like this:
 pub fn total_cmp(&self, other: &Self) -> crate::cmp::Ordering { ... }
 ```
 
-You'll need to change that to a `#[stable]` attribute with a version:
+You'll need to change that to a `#[stable]` attribute with the version set to the placeholder `CURRENT_RUSTC_VERSION`:
 
 ```rust,ignore
-#[stable(feature = "total_cmp", since = "1.61.0")]
+#[stable(feature = "total_cmp", since = "CURRENT_RUSTC_VERSION")]
 ```
 
-Note that, the version number is updated to be the version number of the stable release where this feature will appear. This can be found by consulting [`src/version`](https://github.com/rust-lang/rust/blob/master/src/version) on the current master branch of `rust-lang/rust`.
+Note that other `#[stable]` attributes will contain spelled out version numbers, but you should not spell out any version number as it might get outdated by the time your pull request merges.
 
 ### Remove feature gates from doctests