Document how to override via settings.php

This seems to work for ddev, with a server configuration named "solr":

// Extra settings when using ddev, kept separate to distinguish between what
// ddev added automatically.
if (getenv('IS_DDEV_PROJECT') == 'true' && is_readable($ddev_settings)) {
  // Search API Solr.
  $config['search_api.server.solr']['dependencies']['module'] = ['search_api_solr'];
  $config['search_api.server.solr']['backend'] = 'search_api_solr';
  $config['search_api.server.solr']['backend_config']['connector'] = 'standard';
  $config['search_api.server.solr']['backend_config']['connector_config'] = [
    'scheme' => 'http',
    'host' => 'solr',
    'port' => 8983,
    'path' => '/',
    'core' => 'dev',
    'timeout' => 5,
    'index_timeout' => 5,
    'optimize_timeout' => 10,
    'finalize_timeout' => 30,
    'skip_schema_check' => false,
    'solr_version' => '',
    'http_method' => 'AUTO',
    'commit_within' => '1000',
    'jmx' => false,
    'jts' => false,
    'solr_install_dir' => '',
  ];
}

Note that it didn't work if I didn't set ['dependencies']['module'] and ['backend'], those had to be changed to stop Acquia Search from overriding the configuration.