Clarify docs for RESULT_LARGE_ERR

clippy_lints/src/functions/mod.rs

@@ -252,6 +252,11 @@ declare_clippy_lint! {
     /// A `Result` is at least as large as the `Err`-variant. While we
     /// expect that variant to be seldomly used, the compiler needs to reserve
     /// and move that much memory every single time.
+    /// Furthermore, errors are often simply passed up the call-stack, making
+    /// use of the `?`-operator and its type-conversion mechanics. If the
+    /// `Err`-variant further up the call-stack stores the `Err`-variant in
+    /// question (as library code often does), it itself needs to be at least
+    /// as large, propagating the problem.
     ///
     /// ### Known problems
     /// The size determined by Clippy is platform-dependent.