Address the review of json_view element access

From gregmarr's review of #5622:

- count() counts every member with the key, so count(key) > 1 detects
  duplicate keys; it was contains() as 0/1 before.
- get_to() passes types that get<T>() does not convert directly to
  from_json with the existing value, as basic_json::get_to() does, so that
  a from_json which sets only the fields present keeps the others.
- array_index() checks the empty token and "-" first; the results are
  unchanged.
- Docs: no "if an exception is thrown" where a view never changes
  anything; the borrowed source must also stay unmodified; items() on a
  temporary view is safe; back()/front() notes shortened; the basic_json
  see-also entries say how the view's results can differ.

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

docs/mkdocs/docs/examples/basic_json_view__count.cpp

@@ -5,13 +5,14 @@ using json_document = nlohmann::json_document;
 
 int main()
 {
-    // validate that every transaction of a batch carries a mandatory
+    // validate that every transaction of a batch carries exactly one
     // "amount" field before materializing any of them into a nlohmann::json
-    // value -- count() returns 0 or 1 for an object
+    // value -- count() counts every member with the key
     json_document batch = json_document::parse(R"(
       [
         {"id": 1, "amount": 9.99},
-        {"id": 2}
+        {"id": 2},
+        {"id": 3, "amount": 5.00, "amount": 500.00}
       ]
     )");
 
@@ -19,9 +20,10 @@ int main()
     for (std::size_t i = 0; i < transactions.size(); ++i)
     {
         const auto transaction = transactions[i];
-        if (transaction.count("amount") == 0)
+        const auto amounts = transaction.count("amount");
+        if (amounts != 1)
         {
-            std::cout << "transaction " << i << " is missing \"amount\"\n";
+            std::cout << "transaction " << i << " has " << amounts << " \"amount\" fields\n";
             continue;
         }
         std::cout << "transaction " << i << ": " << transaction["amount"].materialize().dump() << '\n';

docs/mkdocs/docs/examples/basic_json_view__count.output

@@ -1,2 +1,3 @@
 transaction 0: 9.99
-transaction 1 is missing "amount"
+transaction 1 has 0 "amount" fields
+transaction 2 has 2 "amount" fields