Document builder_setter_attrs

I couldn't think of a good example.

derive_builder/src/lib.rs

@@ -504,7 +504,7 @@
 //! # fn main() {}
 //! ```
 //!
-//! ### Additional attributes on fields in the builder
+//! ### Additional attributes on specifically fields in the builder, or the setter function
 //!
 //! You can set other attributes on the field in the builder:
 //!
@@ -528,6 +528,9 @@
 //! # }
 //! ```
 //!
+//! `builder_setter_attrs` works the same way,
+//! but the contained attribute(s) are applied to the generated setter function.
+//!
 //! # **`#![no_std]`** Support (on Nightly)
 //!
 //! You can activate support for `#![no_std]` by adding `#[builder(no_std)]` to your struct