multi: minor coding style and comment clean ups post-discovery merge

This commit implements some minor coding style, commenting and naming
clean up after the recent major discovery service was merged into the
codebase.

Highlights of the naming changes:
* fundingManager.SendToDiscovery -> SendAnnouncement
* discovery.Discovery -> discovery.AuthenticatedGossiper

The rest of the changes consist primary of grammar fixes and proper
column wrapping.

lnwallet/btcwallet/signer.go

@@ -220,9 +220,10 @@ func NewFundingSigner(wallet *BtcWallet) *FundingSigner {
 	}
 }
 
-// SignData sign given data with the private key which corresponds to
-// the given public key.
-// This is a part of the DataSigner interface.
+// SignData sign given data with the private key which corresponds to the given
+// public key.
+//
+// NOTE: This is a part of the DataSigner interface.
 func (s *FundingSigner) SignData(data []byte,
 	pubKey *btcec.PublicKey) (*btcec.Signature, error) {