[Asset] Check that the basePath option is perfectly explained #7872
Description
Recently we made some changes about this to revert some unnoticed BC break introduced a while ago. See symfony/symfony#22526 and symfony/symfony#22528
Some people are confused about the current docs about this feature. See this thread: https://twitter.com/tobyontour/status/859856644272001026 We should explain what happens when the asset contains a leading / and when it doesn't.