Where can I find documentation for icons used in the notification bar? My WiFi icon on the notification bar is displaying an exclamation point and triangles (up/down arrows?). When I try to use Google to find what they mean, there's no clear place documenting notification icons.

I eventually found the exclamation point - can't connect to Google services - but the triangles just aren't searchable.

Where can I find documentation regarding what those sometimes-cryptic notification icons mean?
You can find a list material icons linked in Android Developers which is as official it can be.

But note that OEMs and carriers are free to add / modify their own icons so it's not possible to have all the icons listed on one site officially.