Correct the CC lint's documentation

Author: felix91gr

clippy_lints/src/cognitive_complexity.rs

@@ -14,18 +14,23 @@ use rustc_span::def_id::LocalDefId;
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for methods with high cognitive complexity.
+    /// We used to think it measured how hard a method is to understand.
     ///
     /// ### Why is this bad?
-    /// Methods of high cognitive complexity tend to be hard to
-    /// both read and maintain. Also LLVM will tend to optimize small methods better.
+    /// Ideally, we would like to be able to measure how hard a function is
+    /// to understand given its context (what we call its Cognitive Complexity).
+    /// But that's not what this lint does. See "Known problems"
     ///
     /// ### Known problems
-    /// Sometimes it's hard to find a way to reduce the
-    /// complexity.
+    /// The true Cognitive Complexity of a method is not something we can
+    /// calculate using modern technology. This lint has been left in the
+    /// `nursery` so as to not mislead users into using this lint as a
+    /// measurement tool.
+    ///
+    /// For more detailed information, see [rust-clippy#3793](https://github.com/rust-lang/rust-clippy/issues/3793)
     ///
     /// ### Example
-    /// You'll see it when you get the warning.
+    /// -
     #[clippy::version = "1.35.0"]
     pub COGNITIVE_COMPLEXITY,
     nursery,