Clarify usage of src attribute.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Remove ambiguous statements about escaping from documentation. [`#85`](https://github.com/rust-syndication/atom/pull/85)
 - Update `quick-xml` to `0.37`. [`#86`](https://github.com/rust-syndication/atom/pull/86)
+- Clarify usage of `src` attribute of a `Content` structure. [`#87`](https://github.com/rust-syndication/atom/pull/87)
 
 ## 0.12.4 - 2024-08-28
 

src/content.rs

@@ -12,6 +12,17 @@ use crate::toxml::ToXml;
 use crate::util::{atom_text, atom_xhtml, attr_value, decode};
 
 /// Represents the content of an Atom entry
+//
+/// ## Attention
+///
+/// Atom format specification [RFC4287](https://datatracker.ietf.org/doc/html/rfc4287#section-4.1.3.2)
+/// states that `src` and `value` (content) fields are mutually exclusive:
+///
+/// > atom:content MAY have a "src" attribute, whose value MUST be an IRI reference.
+/// > If the "src" attribute is present, atom:content MUST be empty.
+///
+/// Setting of both fields when authoring an Atom feed is still technically possible,
+/// but it will lead to a non-compliant result.
 #[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
 #[derive(Debug, Default, Clone, PartialEq)]
 #[cfg_attr(feature = "builders", derive(Builder))]