Fix return value of get_ptr for unsigned integers (#4525)

* 🐛 fix return value of get_ptr for unsigned integers

* 📝 update documentation

docs/mkdocs/docs/api/basic_json/get_ptr.md

@@ -35,7 +35,35 @@ Constant.
 
 !!! danger "Undefined behavior"
 
-    Writing data to the pointee of the result yields an undefined state.
+    The pointer becomes invalid if the underlying JSON object changes.
+
+    Consider the following example code where the pointer `ptr` changes after the array is resized. As a result, reading or writing to `ptr` after the array change would be undefined behavior. The address of the first array element changes, because the underlying `std::vector` is resized after adding a fifth element.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json.hpp>
+    
+    using json = nlohmann::json;
+    
+    int main()
+    {
+        json j = {1, 2, 3, 4};
+        auto* ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+    
+        j.push_back(5);
+    
+        ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+    }
+    ```
+
+    Output:
+
+    ```
+    value at 0x6000012fc1c8 is 1
+    value at 0x6000029fc088 is 1
+    ```
 
 ## Examples
 
@@ -54,6 +82,10 @@ Constant.
     --8<-- "examples/get_ptr.output"
     ```
 
+## See also
+
+- [get_ref()](get_ref.md) get a reference value
+
 ## Version history
 
 - Added in version 1.0.0.

docs/mkdocs/docs/api/basic_json/get_ref.md

@@ -40,7 +40,7 @@ Constant.
 
 !!! danger "Undefined behavior"
 
-    Writing data to the referee of the result yields an undefined state.
+    The reference becomes invalid if the underlying JSON object changes.
 
 ## Examples
 
@@ -58,6 +58,10 @@ Constant.
     --8<-- "examples/get_ref.output"
     ```
 
+## See also
+
+- [get_ptr()](get_ptr.md) get a pointer value
+
 ## Version history
 
 - Added in version 1.1.0.