State the exact input size limit of json_document

The check rejects inputs of 0xFFFFFFF0 bytes or more, but the exception
message and the documentation said 4 GiB. Name the limit once
(max_input_size), and state 4 GiB minus 16 bytes in the message and the
documentation. Test the limit with a container that only claims the size.

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

docs/mkdocs/docs/features/json_view.md

@@ -111,7 +111,7 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
 
 - **Only 64-bit integers.** `basic_json_document<BasicJsonType>` requires `BasicJsonType::number_integer_t` and
   `number_unsigned_t` to both be 64 bits wide; this is a compile-time `#!cpp static_assert`.
-- **A 4 GiB input limit.** An input of 4 GiB or more throws
+- **A 4 GiB input limit.** An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws
   [`out_of_range.416`](../home/exceptions.md#jsonexceptionout_of_range416), a limit
   `#!cpp basic_json::parse()` does not have.
 - **A stream is always read to its end.** There is no partial/streaming read of an `#!cpp std::istream`.