Address review: clearer position code and docs

- position_source gets clear(); set_scalar() returns early for new values,
  switches on node_kind, and uses no local copies; the one position_source
  per materialize() call is documented
- no npos + 1 for containers without a position; a comment on why the end
  of a container is reported as its closing bracket
- less duplication under #if in materialize() and the builder
- diagnostic_positions sets npos explicitly
- docs: positions of basic_json values after changes, positions in images
  depend on how the document was parsed, when next is set, and the cost of
  positions while parsing

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

include/nlohmann/detail/view/builder.hpp

@@ -1196,10 +1196,11 @@ indent_done:
 #if JSON_DIAGNOSTIC_POSITIONS
             // the decoded string is in the arena: next keeps the source offset
             // of the opening quote plus one (0: unknown) for materialize()
-            push_node(node_kind::string, node_flags::escaped, 0, r.start, static_cast<std::uint64_t>(r.len) | (static_cast<std::uint64_t>(s - b) << 32));
+            const std::uint64_t second = static_cast<std::uint64_t>(r.len) | (static_cast<std::uint64_t>(s - b) << 32);
 #else
-            push_node(node_kind::string, node_flags::escaped, 0, r.start, r.len);
+            const std::uint64_t second = r.len;
 #endif
+            push_node(node_kind::string, node_flags::escaped, 0, r.start, second);
             return true;
         }
     };