Improve Batch API $context['sandbox'] documentation

Problem/Motivation

The documentation at https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21fo... states (empahsis added):

The 'sandbox' array key is not initially set when this callback is first called, which makes it useful for determining whether it is the first call of the callback or not:

if (empty($context['sandbox'])) {

  // Perform set-up steps here.
}

The words contradict the example.

Steps to reproduce

1. use batch_set in a form to create a batch
2. inspect the incoming $context variable in the operation function

Expected behaviour from description: array_key_exists('sandbox', $context) === false
Current behaviour: array_key_exists('sandbox', $context) === true

Expected behaviour from example: empty($context['sandbox']) === true
Current behaviour: empty($context['sandbox']) === true

Proposed resolution

Update the documentation to indicate the array key is "empty" rather than "not set".

Remaining tasks

User interface changes

None

API changes

Documenttion only

Data model changes

None

Release notes snippet

None