Deprecate or improve documentation for crypto.GenerateEKeyPair

[sukunrt]

In dropping support for go1.20 I came across crypto.GenerateEKeyPair. https://github.com/libp2p/go-libp2p/blob/master/core/crypto/key.go#L129

It looks like no one is using this and some time in the past it was used for SECIO. It also has the cryptic comment Used in the identify module.

@marten-seemann @Stebalien any details you can share about this?

[Stebalien]

Yeah, it was used in secio for forward secrecy. IMO, it can safely be removed and nobody's likely to care.

[Stebalien]

(search of local code + grep.app revealed no users other than secio)

[sukunrt]

fixed by #2711