image and css path field descriptions needs better wording, warn about need for trailing slash

Current text:
The path to a directory for OpenLayers to look for UI graphics. If blank, default graphics are used. Use either a Drupal path (and the appropriate base path will be prepended), an absolute path (such as /path/to/images/), or a full URL.

New text:
The path to a directory of UI graphics. Leave blank for default graphics. Use either relative path (without leading slash, base path will be prepended, f. ex. "sites/all/themes//
/"), absolute path (with leading slash, path will start from sites root directory, f. ex. "/path/to/images/) or a full URL. Always include trailing slash.