Address the review of editable json documents

From gregmarr's review of #5630:

- set() with a JSON pointer below a null value checks for "-" before
  scanning the token for digits.
- node_flags::storage is defined as escaped | edited, and the comment on
  document_data::base says that the fourth slot is unused.
- source_extent() names its root check.
- The integer-index overloads of set(), insert(), and erase() use a named
  trait instead of a negated condition, which astyle formats like the
  library's other enable_ifs.

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

include/nlohmann/detail/view/document_data.hpp

@@ -54,7 +54,7 @@ struct document_data
     std::vector<object_index> indexes;
     std::vector<std::uint32_t> index_slots;
     std::vector<std::uint32_t> large_objects; ///< positions of the objects to index (noted while parsing)
-    std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena, edit arena (indexed by flags & node_flags::storage)
+    std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena, edit arena, unused (indexed by flags & node_flags::storage)
     bool discarded = true;
 
     /// The storage of edits (editable documents only; see edit_storage.hpp).