Make clearer how to use prettyplease (#2468)

Author: pvdrz

CHANGELOG.md

@@ -164,9 +164,13 @@
 ## Added
  * Added the `Bindgen::default_visibility` nethod and the
    `--default-visibility` flag to set the default visibility of fields.
- * Added the `--formatter` flag with the values `none`, `rustfmt` and
+ * Added the `--formatter` CLI flag with the values `none`, `rustfmt` and
    `prettyplease` to select which tool will be used to format the bindings. The
    default value is `rustfmt`. 
+ * Added the `Builder::formatter` method and the `Formatter` type to select
+   which tool will be used to format the bindings. The
+   `Formatter::Prettyplease`   variant is only available if the
+   `"prettyplease"` feature is enabled. 
 ## Changed
  * Static functions with no arguments use `void` as their single argument
    instead of having no arguments when the `--wrap-static-fns` flag is used.

bindgen/lib.rs

@@ -1667,6 +1667,9 @@ impl Builder {
     }
 
     /// Set which tool should be used to format the generated bindings.
+    ///
+    /// To be able to choose `prettyplease` as a formatter, the `"prettyplease"` feature must be
+    /// enabled.
     pub fn formatter(mut self, formatter: Formatter) -> Self {
         self.options.formatter = formatter;
         self