Add description for dst parameter of bson_value_copy (#1874)

`dst` should be uninitialized to prevent memory leak

Signed-off-by: linraymond2006 <[email protected]>

Author: LinRaymond2006

src/libbson/doc/bson_value_copy.rst

@@ -21,6 +21,7 @@ Description
 -----------
 
 This function will copy the boxed content in ``src`` into ``dst``. ``dst`` must be freed with :symbol:`bson_value_destroy()` when no longer in use.
+``dst`` *MUST* be an uninitialized :symbol:`bson_value_t` to avoid leaking memory.
 
 .. note::