docs: add release notes for non-Rust structured_sources

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: bonzini

docs/markdown/snippets/structured-sources-any-language.md

@@ -0,0 +1,11 @@
+## `structured_sources()` can be used with any language
+
+[[structured_sources]] previously only worked with Rust build targets
+and Java resources, and raised an error otherwise. It can now be used
+with a target of any language (other than Java, where it can only be
+used for resources like before).
+
+Files will be laid out in the build directory according to the structure
+described by the dictionary keys; for example, in the case of C
+it will be possible to include a file from the `structured_sources()`
+from another according to their provided path.