Clarify distinction between Drop trait and drop function. (#1255)

This clears up some misleading explanation from #246.

src/traits/drop.md

@@ -31,9 +31,16 @@ fn main() {
 
 <details>
 
-* `drop` is called automatically, but it can be called manually like in this example.
-* If it was called manually, it won't be called at the end of the scope for the second time.
-* Calling `drop` can be useful for objects that do some work on `drop`: releasing locks, closing files, etc.
+* Note that `std::mem::drop` is not the same as `std::ops::Drop::drop`.
+* Values are automatically dropped when they go out of scope.
+* When a value is dropped, if it implements `std::ops::Drop` then its `Drop::drop` implementation
+  will be called.
+* All its fields will then be dropped too, whether or not it implements `Drop`.
+* `std::mem::drop` is just an empty function that takes any value. The significance is that it takes
+  ownership of the value, so at the end of its scope it gets dropped. This makes it a convenient way
+  to explicitly drop values earlier than they would otherwise go out of scope.
+    * This can be useful for objects that do some work on `drop`: releasing locks, closing files,
+      etc.
 
 Discussion points: