Modify rule S2807: Add a longer explanation of the hidden friend pattern

rules/S2807/cfamily/rule.adoc

@@ -3,7 +3,23 @@ This rule raises issues for overloaded binary mathematical and relational operat
 == Why is this an issue?
 
 When overloading binary or relational operators, it is recommended that they be declared hidden friends of the class.
-A hidden friend, i.e. a friend function defined in the class body, provides the following benefits:
+
+=== The hidden friend pattern
+
+The hidden friend pattern consists of declaring and defining a function directly as a friend inside the class body. This reduces the function's visibility to argument-dependent lookup only. Approximately, such a function is considered only when called on an object of the enclosing class.
+
+[source,cpp]
+----
+struct MyClass {
+  friend void function(MyClass const& arg) { // This function is a hidden friend of MyClass
+    ...
+  }
+};
+----
+
+=== Benefits of hidden friends
+
+Using hidden friends provides the following benefits:
 
 * in contrast to the member function, it allows conversion to be applied to both operands
 * in contrast to free functions, it is considered only if one of the operands is an object of the given class