add patch_inplace function (#3581)

* add patch_inplace function to json class

* add documentation

* fix up docs

docs/examples/patch_inplace.cpp

@@ -0,0 +1,34 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // the original document
+    json doc = R"(
+        {
+          "baz": "qux",
+          "foo": "bar"
+        }
+    )"_json;
+
+    // the patch
+    json patch = R"(
+        [
+          { "op": "replace", "path": "/baz", "value": "boo" },
+          { "op": "add", "path": "/hello", "value": ["world"] },
+          { "op": "remove", "path": "/foo"}
+        ]
+    )"_json;
+
+    // output original document
+    std::cout << "Before\n" << std::setw(4) << doc << std::endl;
+
+    // apply the patch
+    doc.patch_inplace(patch);
+
+    // output patched document
+    std::cout << "After\n" << std::setw(4) << doc << std::endl;
+}