Fix the capture checking documentation

The syntax on the official Scala page was outdated.
Also, new docs nudge people to use the latest version of Scala,
which allow trying capture checking out on a stable compiler version.

[Cherry-picked 97138fb]

Author: abgruszecki

docs/_docs/reference/experimental/cc.md

@@ -8,7 +8,8 @@ Capture checking is a research project that modifies the Scala type system to tr
 ```scala
 import language.experimental.captureChecking
 ```
-At present, capture checking is still highly experimental and unstable.
+At present, capture checking is still highly experimental and unstable, and it evolves quickly.
+Before trying it out, make sure you have the latest version of Scala.
 
 To get an idea what capture checking can do, let's start with a small example:
 ```scala