Document extra behavior of #[no_mangle]

Author: jonas-schievink

src/abi.md

@@ -63,6 +63,9 @@ The *`no_mangle` attribute* may be used on any [item] to disable standard
 symbol name mangling. The symbol for the item will be the identifier of the
 item's name.
 
+Additionally, the item will be publicly exported from the produced library or
+object file, similar to `#[used]`.
+
 ## The `link_section` attribute
 
 The *`link_section` attribute* specifies the section of the object file that a