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>

docs/mkdocs/docs/api/basic_json_document/root.md

@@ -1,10 +1,15 @@
 # <small>nlohmann::basic_json_document::</small>root
 
 ```cpp
-view_type root() const noexcept;
+// (1)
+view_type root() const& noexcept;
+
+// (2)
+view_type root() const&& = delete;
 ```
 
-Returns a view of the root value of the document.
+1. Returns a view of the root value of the document.
+2. Deleted: the view of a temporary document would dangle.
 
 ## Return value
 
@@ -21,6 +26,16 @@ Constant.
 
 ## Notes
 
+**Lifetime.** A view refers into the document, so the document must outlive it. `root()` can therefore only be called
+on a document that has a name (an lvalue); calling it on a temporary does not compile:
+
+```cpp
+auto v = json_document::parse(text).root();  // error: the document is destroyed at the end of the statement
+
+auto doc = json_document::parse(text);       // OK: keep the document alive
+auto v = doc.root();
+```
+
 `root()` is a cheap handle into the document's index, not a copy of anything; call it as often as needed. The
 returned view is valid under the same conditions as any other view of the document -- see
 [Object inspection](../basic_json_view/index.md) -- in particular, it is invalidated by the next

docs/mkdocs/docs/features/json_view.md

@@ -76,6 +76,9 @@ Moving the document itself is fine and does **not** invalidate its views: the in
 that keeps its address across the move. Take a fresh view from [`root()`](../api/basic_json_document/root.md)
 whenever any of the other conditions above was not met.
 
+Because a view dies with its document, [`root()`](../api/basic_json_document/root.md) is not callable on a temporary
+document: `#!cpp auto v = json_document::parse(text).root();` does not compile. Give the document a name first.
+
 ??? example "Example: borrowed and owned documents, and when views become invalid"
 
     ```cpp