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/home/architecture.md

@@ -210,7 +210,8 @@ packet-beta
 - **Navigation** needs no pointers: the elements of an array or object follow its node, and the node after a value's
   subtree is `next` nodes further for an array or object, and the next node otherwise (`document_data::after`). Views
   step from element to element this way and skip whole subtrees in constant time.
-- **Offsets** are 32 bits wide, so a document is limited to 4 GiB (`out_of_range.416`).
+- **Offsets** are 32 bits wide, so a document is limited to 4294967279 bytes, 4 GiB minus 16 bytes (a margin below
+  2^32 for positions one scanner step past the end of the text; `out_of_range.416`).
 
 For example, `#!json {"a": [1, 2.5]}` becomes five nodes. Each node's elements follow it, and `next` leads from an
 array or object past its subtree:

docs/mkdocs/docs/home/exceptions.md

@@ -1066,12 +1066,12 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 
 [`basic_json_document::parse()`](../api/basic_json_document/parse.md) and the other parsing functions of
 [`basic_json_document`](../api/basic_json_document/index.md) index a value's position in the source text in 32 bits,
-so they do not support an input of 4 GiB or more.
+so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more.
 
 !!! failure "Example message"
 
     ```
-    [json.exception.out_of_range.416] input of 4 GiB or more is not supported by json_document
+    [json.exception.out_of_range.416] input of 4294967280 bytes or more is not supported by json_document
     ```
 
 !!! note