Explain the syntax of stability flags

doc/04-schema.md

@@ -276,10 +276,11 @@ Example:
 
 All links are optional fields.
 
-`require` and `require-dev` additionally support stability flags ([root-only](04-schema.md#root-package)).
+`require` and `require-dev` additionally support _stability flags_ ([root-only](04-schema.md#root-package)).
+They take the form "_constraint_@_stability flag_". 
 These allow you to further restrict or expand the stability of a package beyond
 the scope of the [minimum-stability](#minimum-stability) setting. You can apply
-them to a constraint, or apply them to an empty constraint if you want to
+them to a constraint, or apply them to an empty _constraint_ if you want to
 allow unstable packages of a dependency for example.
 
 Example: