Delete json_document::root() on temporary documents

auto v = json_document::parse(text).root() compiled and left the view
dangling. Delete the overload for rvalue documents, take a named document
in the tests, and document the lifetime rule.

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

include/nlohmann/json_view.hpp

@@ -358,7 +358,7 @@ class basic_json_document
     ////////////
 
     /// the root value (discarded if parsing failed without exceptions)
-    view_type root() const noexcept
+    view_type root() const& noexcept
     {
         if (!m_data || m_data->discarded)
         {
@@ -367,6 +367,9 @@ class basic_json_document
         return view_type(m_data.get(), m_data->tape);
     }
 
+    /// deleted: the view of a temporary document would dangle
+    view_type root() const&& = delete;
+
     bool is_discarded() const noexcept
     {
         return !m_data || m_data->discarded;