Document that "versions" don't override "version arguments" when detecting libraries [#2840423]

Reading about "versions" in libraries/libraries.api.php:

Each value is an associative array of top-level properties that are entirely overridden by the version.

However, this isn't the case when detecting libraries - only the top level "version arguments" are used.

The work-around is to create a version callback function that first check top level "version arguments" and then all the versions "version arguments":

function mymodule_libraries_get_version($library, $options) {
  // Detect the version using library defaults.
  $version = libraries_get_version($library, $options);

  if (!empty($version)) {
    return $version;
  }

  // Sort versions in descending order (newest first).
  krsort($library['versions']);

  // Detect the version using version specific overrides.
  foreach ($library['versions'] as $version_properties) {
    if (isset($version_properties['version arguments'])) {
      $options = $version_properties['version arguments'] += $options;
      $version = libraries_get_version($library, $options);

      if (!empty($version)) {
        return $version;
      }
    }
  }
}