Add note on derived return type for value function (#4628)

* 📡 add note on derived return type

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📡 add note on derived return type

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/examples/value__return_type.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    json j = json::parse(R"({"uint64": 18446744073709551615})");
+
+    std::cout << "operator[]:                " << j["uint64"] << '\n'
+              << "default value (int):       " << j.value("uint64", 0) << '\n'
+              << "default value (uint64_t):  " << j.value("uint64", std::uint64_t(0)) << '\n'
+              << "explict return value type: " << j.value<std::uint64_t>("uint64", 0) << '\n';
+}

docs/mkdocs/docs/examples/value__return_type.output

@@ -0,0 +1,4 @@
+operator[]:                18446744073709551615
+default value (int):       -1
+default value (uint64_t):  18446744073709551615
+explict return value type: 18446744073709551615