Address the second review round of json_view element access

From gregmarr's review of #5622:

- begin()/end(): "a null value or a discarded view iterates as an empty
  range", singular like the primitive case before it.
- The class page's list of function groups has no trailing commas.
- document_data::key_equals() compares a key node with a string, for the
  key comparisons that were spelled out in each lookup.

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

docs/mkdocs/docs/api/basic_json_view/begin.md

@@ -6,7 +6,7 @@ iterator begin() const noexcept;
 
 Returns an iterator to the first element of an array, or the first member value of an object, in **document order**
 -- the order the values appear in the source text, not sorted by key. A primitive value iterates as a range of one
-element (itself); `#!json null` and a [discarded](is_discarded.md) view iterate as an empty range.
+element (itself), and a `#!json null` value or a [discarded](is_discarded.md) view iterates as an empty range.
 
 ## Return value