clang doesn't document the full list of warning flags that it supports, but the clang source code has a list, including the hierarchy of meta flags that enable other flags:
http://fuckingclangwarnings.com/ has descriptions of many of the warnings, but more recent warnings aren't covered because the website hasn't been updated since 2014.
Thank you! I've never seen that page. It doesn't show up when searching for "clang warnings" in DuckDuckGo or Bing, but I see now that it is Google's result #2.
I just found a GitHub repo that lists which warning flags are supported by different versions of clang and gcc: