xso: mark iterator type as non-breaking to change

Just like with the builder type, the concrete iterator type on IntoXml
is supposed to be an implementation detail. That allows switching freely
between various ways to generate such a type.

xso/src/lib.rs

@@ -27,8 +27,18 @@ pub mod exports {
 
 /// Trait allowing to consume a struct and iterate its contents as
 /// serialisable [`rxml::Event`] items.
+///
+/// **Important:** Changing the [`EventIter`][`Self::EventIter`] associated
+/// type is considered a non-breaking change for any given implementation of
+/// this trait. Always refer to a type's iterator type using fully-qualified
+/// notation, for example: `<T as xso::IntoXml>::EventIter`.
 pub trait IntoXml {
     /// The iterator type.
+    ///
+    /// **Important:** Changing this type is considered a non-breaking change
+    /// for any given implementation of this trait. Always refer to a type's
+    /// iterator type using fully-qualified notation, for example:
+    /// `<T as xso::IntoXml>::EventIter`.
     type EventIter: Iterator<Item = Result<rxml::Event, self::error::Error>>;
 
     /// Return an iterator which emits the contents of the struct or enum as