json_view: node_kind in the node writers, spell out read_only, comment fixes

The builder and the edit functions take and store a node_kind instead of
casting a value_t; the numbering assertions and the comments now speak of
node_kind, and the link kind notes that it follows value_t::discarded.
The read-only aliases name view_mode::read_only like the editable ones.
view_mode.md says the mode is a compile-time property of the type.

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

docs/mkdocs/docs/api/view_mode.md

@@ -30,7 +30,7 @@ The shortcuts name both modes: [`json_document`](json_document.md) and [`json_vi
 
 ## Notes
 
-The mode is part of the view's type, and there is no conversion between the two modes: a read-only view reads a
+The mode is a compile-time property of the document's and the view's type, not a setting of a call, and there is no conversion between the two modes: a read-only view reads a
 container in place, while a view of an editable document follows the links that edits leave behind. A function that
 accepts views therefore names the modes it accepts in its signature: