docs: Add missing docstrings, adjust wording to match standard style

This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.

In some cases, the identifiers were un-exported after noticing they had
no external users.

Besides improving documentation, it also reduces the linter noise
significantly.

internal/courier/smtp.go

@@ -41,6 +41,8 @@ type SMTP struct {
 	Dinfo *domaininfo.DB
 }
 
+// Deliver an email. On failures, returns an error, and whether or not it is
+// permanent.
 func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
 	a := &attempt{
 		courier:  s,