Document how images store the node index of json_view

The node index section on the architecture page says how save() writes
the nodes and that a change of their layout must raise the image
version, and save's format note links to it.

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

docs/mkdocs/docs/api/basic_json_document/save.md

@@ -58,9 +58,10 @@ up in the image.
 ## Notes
 
 **Format.** The image begins with a 64-byte header (the magic bytes `#!cpp "NJVI"`, a version number, the node count,
-and the sizes of the text and the decoded strings, all little-endian), followed by the nodes (16 bytes each), the
-text and a `#!cpp '\0'`, and the decoded strings and a `#!cpp '\0'`. [`load`](load.md) checks the header, and the
-sizes it describes, before reading anything else -- see [`load`'s Exceptions](load.md#exceptions).
+and the sizes of the text and the decoded strings, all little-endian), followed by the nodes
+([16 bytes each](../../home/architecture.md#node-index-of-json-views)), the text and a `#!cpp '\0'`, and the decoded
+strings and a `#!cpp '\0'`. [`load`](load.md) checks the header, and the sizes it describes, before reading anything
+else -- see [`load`'s Exceptions](load.md#exceptions).
 
 !!! warning "Experimental"
 

docs/mkdocs/docs/home/architecture.md

@@ -259,6 +259,14 @@ edited:
 - Views of read-only documents compile without any of this: how views walk the index is a template parameter
   (`navigation<Editable>`).
 
+Images ([`save`](../api/basic_json_document/save.md) and [`load`](../api/basic_json_document/load.md),
+[`detail/view/image.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/image.hpp)) store
+the nodes as they are: a 64-byte header (the magic bytes `NJVI`, a format version, the sizes, and reserved bytes that
+must be zero), the nodes, the text, and the decoded strings. An edited document is first written in document order, as
+the parser would have written it (without links), and the numbers of the hash indexes are cleared, since `load`
+rebuilds the indexes. So a change of the node layout is a change of the image format: it must raise `image_version`,
+and `load` then rejects images of other versions (`parse_error.116`) instead of misreading them.
+
 ## Input adapters
 
 Input is read via **input adapters** that abstract a source. Every input adapter provides this interface: