Refactor comments for clarity and detail

Enhanced the descriptive comments for error variables, constants, and types to provide clearer and more detailed explanations. This improves code readability and ensures that the purpose and usage of different elements are better understood by developers.

msg.go

@@ -24,51 +24,60 @@ import (
 )
 
 var (
-	// ErrNoFromAddress should be used when a FROM address is requrested but not set
+	// ErrNoFromAddress indicates that the FROM address is not set, which is required.
 	ErrNoFromAddress = errors.New("no FROM address set")
 
-	// ErrNoRcptAddresses should be used when the list of RCPTs is empty
+	// ErrNoRcptAddresses indicates that no recipient addresses have been set.
 	ErrNoRcptAddresses = errors.New("no recipient addresses set")
 )
 
 const (
-	// errTplExecuteFailed is issued when the template execution was not successful
+	// errTplExecuteFailed indicates that the execution of a template has failed, including the underlying error.
 	errTplExecuteFailed = "failed to execute template: %w"
 
-	// errTplPointerNil is issued when a template pointer is expected but it is nil
+	// errTplPointerNil indicates that a template pointer is nil, which prevents further template execution or
+	// processing.
 	errTplPointerNil = "template pointer is nil"
 
-	// errParseMailAddr is used when a mail address could not be validated
+	// errParseMailAddr indicates that parsing of a mail address has failed, including the problematic address
+	// and error.
 	errParseMailAddr = "failed to parse mail address %q: %w"
 )
 
 const (
-	// NoPGP indicates that a message should not be treated as PGP encrypted
+	// NoPGP indicates that a message should not be treated as PGP encrypted or signed and is the default value
-	// or signed and is the default value for a message
+	// for a message
 	NoPGP PGPType = iota
-	// PGPEncrypt indicates that a message should be treated as PGP encrypted
+	// PGPEncrypt indicates that a message should be treated as PGP encrypted. This works closely together with
-	// This works closely together with the corresponding go-mail-middleware
+	// the corresponding go-mail-middleware.
 	PGPEncrypt
-	// PGPSignature indicates that a message should be treated as PGP signed
+	// PGPSignature indicates that a message should be treated as PGP signed. This works closely together with
-	// This works closely together with the corresponding go-mail-middleware
+	// the corresponding go-mail-middleware.
 	PGPSignature
 )
 
-// MiddlewareType is the type description of the Middleware and needs to be returned
+// MiddlewareType is a type wrapper for a string. It describes the type of the Middleware and needs to be
-// in the Middleware interface by the Type method
+// returned by the Middleware.Type method to satisfy the Middleware interface.
 type MiddlewareType string
 
-// Middleware is an interface to define a function to apply to Msg before sending
+// Middleware represents the interface for modifying or handling email messages. A Middleware allows the user to
+// alter a Msg before it is finally processed. Multiple Middleware can be applied to a Msg.
+//
+// Type returns a unique MiddlewareType. It describes the type of Middleware and makes sure that
+// a Middleware is only applied once.
+// Handle performs all the processing to the Msg. It always needs to return a Msg back.
 type Middleware interface {
 	Handle(*Msg) *Msg
 	Type() MiddlewareType
 }
 
-// PGPType is a type alias for a int representing a type of PGP encryption
+// PGPType is a type wrapper for an int, representing a type of PGP encryption or signature.
-// or signature
 type PGPType int
 
-// Msg is the mail message struct
+// Msg represents an email message with various headers, attachments, and encoding settings.
+//
+// The Msg is the central part of go-mail. It provided a lot of methods that you would expect in a mail
+// user agent (MUA). Msg satisfies the io.WriterTo and io.Reader interfaces.
 type Msg struct {
 	// addrHeader is a slice of strings that the different mail AddrHeader fields
 	addrHeader map[AddrHeader][]*mail.Address