[spec/expression] Add struct equality example (#4219)

Also add 2 links.

Author: ntrel

spec/expression.dd

@@ -686,6 +686,21 @@ $(GNAME EqualExpression):
         results of the corresponding object fields.
     )
 
+    $(SPEC_RUNNABLE_EXAMPLE_RUN
+    ---
+    struct S
+    {
+        int i = 4;
+        string s = "four";
+    }
+
+    S s;
+    assert(s == S());
+    s.s = "foul";
+    assert(s != S());
+    ---
+    )
+
         $(IMPLEMENTATION_DEFINED The contents of any $(DDSUBLINK spec/struct, struct_layout,
         alignment gaps) in the struct object.)
 

spec/struct.dd

@@ -2534,7 +2534,9 @@ void main()
 ---
 )
 
-        $(P If a struct declaration defines an $(D opCmp) or $(D opEquals)
+        $(P If a struct declaration defines an
+        $(DDSUBLINK spec/operatoroverloading, compare, `opCmp`) or
+        $(DDSUBLINK spec/operatoroverloading, equals, `opEquals`)
         method, it will take precedence to that of the *AliasThis* member. Note
         that, unlike an $(D opCmp) method, an $(D opEquals) method is implicitly
         defined for a $(D struct) declaration if a user-defined one isn't provided.