Clarify ed25519 signatures on routerdescs.

       It MUST be the next-to-last element in the descriptor, appearing

       immediately before the RSA signature. It MUST contain an Ed25519

       signature of a SHA256 digest of the entire document, from the

       first character up to and including the first space after the

       "router-sig-ed25519" string, prefixed with the string "Tor

       router descriptor signature v1".  Its format is:

       signature of a SHA256 digest of the entire document. This digest is

       taken from the first character up to and including the first space

       after the "router-sig-ed25519" string. Before computing the digest,

       the string "Tor router descriptor signature v1" is prefixed to the

       document.

       The signature is encoded in Base64 with terminating =s removed.

       The signature is encoded in Base64, with terminating =s removed.

       The signing key in the identity-ed25519 certificate MUST

       be the one used to sign the document.