Note that features can't be stabilized until they go through FCP

... and that FCP happens on the tracking issue, not the PR.

Author: jyn514

src/stability.md

@@ -72,6 +72,7 @@ Furthermore this attribute is needed to mark an intrinsic as callable from
 
 To stabilize a feature, follow these steps:
 
+0. Ask a **@T-libs** to start an FCP on the tracking issue and wait for the FCP to complete (with `disposition-merge`).
 1. Change `#[unstable]` to `#[stable(since = "version")]`.
    `version` should be the *current nightly*, i.e. stable+2. You can see which version is
    currently nightly [on Forge](https://forge.rust-lang.org/#current-release-versions).