Clarify how needs_drop is conservative

Author: Gankra

src/libcore/mem.rs

@@ -429,9 +429,11 @@ pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
 
 /// Returns whether dropping values of type `T` matters.
 ///
-/// This is purely an optimization hint, and may be implemented conservatively.
-/// For instance, always returning `true` would be a valid implementation of
-/// this function.
+/// This is purely an optimization hint, and may be implemented conservatively:
+/// it may return `true` for types that don't actually need to be dropped.
+/// As such always returning `true` would be a valid implementation of
+/// this function. However if this function actually returns `false`, then you
+/// can be certain dropping `T` has no side effect.
 ///
 /// Low level implementations of things like collections, which need to manually
 /// drop their data, should use this function to avoid unnecessarily