This is a fairly common attribute on methods, but is not something you
need to know when reading the method docs - the purpose of the attribute
is for the compiler to tell you about it if you forget to use a value.
Removing reclaims some valuable space in the summary of methods.
Currently attributes are only shown for structs, unions and enums but
they should be shown for all items. For example it is useful to know if a
function is `#[no_mangle]`.
