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>

include/nlohmann/detail/view/node.hpp

@@ -29,6 +29,11 @@ static_assert(static_cast<std::uint8_t>(value_t::null) == 0 && static_cast<std::
               && static_cast<std::uint8_t>(value_t::number_unsigned) == 6 && static_cast<std::uint8_t>(value_t::number_float) == 7,
               "the node format depends on the numbering of value_t");
 
+/// The largest input a document accepts, in bytes. Offsets and node counts are
+/// 32 bits wide; the limit keeps 16 bytes (the width of the scanner's steps)
+/// below 2^32, so that a position one step past the end of the text fits.
+static constexpr std::size_t max_input_size = 0xFFFFFFEFu;
+
 /// node flags
 struct node_flags
 {