docs: Describe Origin::primary

Author: epage

src/snippet.rs

@@ -413,6 +413,10 @@ impl<'a> Origin<'a> {
         self
     }
 
+    /// Mark this origin as the primary one within this [`Group`]
+    ///
+    /// Generally there should only be one primary origin, whether from an [`Origin`] or an
+    /// [`Annotation`] with [`AnnotationKind::Primary`].
     pub fn primary(mut self, primary: bool) -> Self {
         self.primary = primary;
         self