crypto/tls: generate_cert.go should document that some ECDSA curves don't have browser support

[kevinburke]

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.

[aead]

I would recommend to not use P512 and instead use X25519, P256 or P224 because the Go TLS package does not provide/use a constant time implementation of P384 or P512 yet. Using P384 or P512 can enable side-channel attacks.