docs: writing generic code for json_view (#5924)

* docs: writing generic code for json_view

A "Writing generic code" section on the json_view page: why a function
taking json_view accepts no other view, the three signatures for
editable, read-only, or either kind of view (passed by value), why the
modes do not convert, is_json_view, and how a from_json() reads views
(constrained to json values and views; the macros and the library's
conversions do it already) with what differs then (duplicate keys,
JSON_DIAGNOSTICS paths).

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

* docs: name the documents the generic view function accepts

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

* docs: views of read-only documents convert to editable views

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

---------

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

docs/mkdocs/docs/api/view_mode.md

@@ -50,7 +50,7 @@ void f(basic_json_view<BasicJsonType, view_mode::editable> v);
 template<typename BasicJsonType>
 void f(basic_json_view<BasicJsonType, view_mode::read_only> v);
 
-// either
+// views of either document type
 template<typename BasicJsonType, view_mode Mode>
 void f(basic_json_view<BasicJsonType, Mode> v);
 ```
@@ -82,6 +82,7 @@ nor a [`json_editable_view`](json_editable_view.md). Views of the two modes comp
 - [basic_json_view](basic_json_view/index.md) - a handle to one value of a document
 - [json_editable_document](json_editable_document.md) - an editable document of `json`
 - [(constructor)](basic_json_view/basic_json_view.md) - the conversion from a read-only to an editable view
+- [Writing generic code](../features/json_view.md#writing-generic-code) - functions that accept views
 
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