Update rust book cfg chapter

src/doc/unstable-book/src/compiler-flags/check-cfg.md

@@ -77,10 +77,11 @@ Those well known names and values follows the same stability as what they refer
 Well known names and values checking is always enabled as long as at least one
 `--check-cfg` argument is present.
 
-As of `2024-01-09T`, the list of known names is as follows:
+As of `2024-02-15T`, the list of known names is as follows:
 
 <!--- See CheckCfg::fill_well_known in compiler/rustc_session/src/config.rs -->
 
+ - `clippy`
  - `debug_assertions`
  - `doc`
  - `doctest`