more documentation

In this commit, also the semantics for values skipped via the parser
callback has changed. Now, the top-level value is returned as “null”
instead of “discarded”.

doc/examples/swap__reference.cpp

@@ -0,0 +1,17 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create two JSON values
+    json j1 = {1, 2, 3, 4, 5};
+    json j2 = {{"pi", 3.141592653589793}, {"e", 2.718281828459045}};
+
+    // swap the values
+    j1.swap(j2);
+
+    // output the values
+    std::cout << "j1 = " << j1 << '\n';
+    std::cout << "j2 = " << j2 << '\n';
+}