Add more descriptive details

Author: notriddle

clippy_lints/src/drop_bounds.rs

@@ -12,6 +12,14 @@ use rustc::{declare_tool_lint, lint_array};
 /// `Drop::drop`, and that function is by fiat not callable in user code.
 /// So there is really no use case for using `Drop` in trait bounds.
 ///
+/// The most likely use case of a drop bound is to distinguish between types
+/// that have destructors and types that don't. Combined with specialization,
+/// a naive coder would write an implementation that assumed a type could be
+/// trivially dropped, then write a specialization for `T: Drop` that actually
+/// calls the destructor. Except that doing so is not correct; String, for
+/// example, doesn't actually implement Drop, but because String contains a
+/// Vec, assuming it can be trivially dropped will leak memory.
+///
 /// **Known problems:** None.
 ///
 /// **Example:**