[doc]: NVS documentation updates

* Better comment of nvs_get_used_entry_count()
* Mention C++ example in API reference
* WIP: Used target instead of hard code ESP32
* Note that strings can only span one page
* Reflect that item types have been moved
* Some clarification about nvs_commit()
* Improved reference to the ESP Partition API
* fixed little mistake in documenting-code.rst
* Change of nvs_open_from_part() to
  nvs_open_from_partition() reflected in docs
* Corrected documentation of
  NVSHandle::get_string(), NVSHandle::get_blob()
  and NVSHandle::get_item_size().

* Closes IDF-1563
* Closes IDF-859
* Closes https://github.com/espressif/esp-idf/issues/6123

docs/en/api-reference/storage/nvs_flash.rst

@@ -27,6 +27,10 @@ You can find two code examples in the :example:`storage` directory of ESP-IDF ex
 
   The example also shows how to implement the diagnostic procedure to check if the read / write operation was successful.
 
+:example:`storage/nvs_rw_value_cxx`
+
+  This example does exactly the same as :example:`storage/nvs_rw_value`, except that it uses the C++ nvs handle class.
+
 
 API Reference
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