fix(dev): update device ID documentation to Ed25519 (fixes #1028) (#1029)

Syncthing uses ed25519 since v2.0.0, but this was not updated in the
docs. Solves #1028

I only updated the one important paragraph, but maybe it's better to
explain what algos can actually be used and update the example output of
`openssl x509` with an actual certificate generated in the latest
version?

Author: girlf4ngs

dev/device-ids.rst

@@ -14,12 +14,12 @@ Keys
 To understand device IDs we need to look at the underlying mechanisms. At first
 startup, Syncthing will create a public/private keypair.
 
-Currently this is a 384 bit ECDSA key (3072 bit RSA prior to v0.12.5,
-which is what is used as an example in this article). The keys are saved in
-the form of the private key (``key.pem``) and a self signed certificate
-(``cert.pem``). The self signing part doesn't actually add any security or
-functionality as far as Syncthing is concerned but it enables the use of the
-keys in a standard TLS exchange.
+Currently this is a 256 bit Ed25519 key (384 bit ECDSA prior to v2.0.0 and
+3072 bit RSA prior to v0.12.5, which is what is used as an example in this
+article). The keys are saved in the form of the private key (``key.pem``) and a
+self signed certificate (``cert.pem``). The self signing part doesn't actually
+add any security or functionality as far as Syncthing is concerned but it enables
+the use of the keys in a standard TLS exchange.
 
 The typical certificate will look something like this, inspected with
 ``openssl x509``::