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>