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/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