Show details in cfg version unstable book

Author: pickfire

src/doc/unstable-book/src/language-features/cfg-version.md

@@ -7,19 +7,20 @@ The tracking issue for this feature is: [#64796]
 ------------------------
 
 The `cfg_version` feature makes it possible to execute different code
-depending on the compiler version.
+depending on the compiler version. It will return true if the compiler
+version is greater than or equal to the specified version.
 
 ## Examples
 
 ```rust
 #![feature(cfg_version)]
 
-#[cfg(version("1.42"))]
+#[cfg(version("1.42"))] // 1.42 and above
 fn a() {
     // ...
 }
 
-#[cfg(not(version("1.42")))]
+#[cfg(not(version("1.42")))] // 1.41 and below
 fn a() {
     // ...
 }