Make it clearer that the private implementation details are private

Both of these modules are doc(hidden) and commented "Not public API",
but incorrect downstream code sometimes still references them. Naming
the module __private will make it more likely to be noticed in code
review.

serde/src/ser/mod.rs

@@ -711,7 +711,7 @@ pub trait Serializer: Sized {
     ///
     /// ```edition2018
     /// # use serde::ser::{Serializer, SerializeSeq};
-    /// # use serde::private::ser::Error;
+    /// # use serde::__private::ser::Error;
     /// #
     /// # struct MySerializer;
     /// #