crypto/tls: generate_cert.go should document that some ECDSA curves don't have browser support #19901
Description
I ran generate_cert.go and figured the highest numbered ECDSA curve (P521) was the best one. It turns out that curve isn't supported in the latest versions of Chrome, Chromium and Firefox, which (as best as I can tell) only support P256 and P384.
If you try to connect using those certificates, you get a confusing remote error: tls: illegal parameter error message.
See also #19900.