Document non-generation of size_t definition (#2607)

* Document non-generation of `size_t` definition

* Run rustfmt

Author: pvdrz

bindgen/options/mod.rs

@@ -1589,6 +1589,9 @@ options! {
         methods: {
             /// Set whether `size_t` should be translated to `usize`.
             ///
+            /// If `size_t` is translated to `usize`, type definitions for `size_t` will not be
+            /// emitted.
+            ///
             /// `size_t` is translated to `usize` by default.
             pub fn size_t_is_usize(mut self, is: bool) -> Self {
                 self.options.size_t_is_usize = is;