Clarify Msg struct field comments

Updated comments for all fields in the Msg struct to provide clearer and more detailed explanations. This includes specifying the data type for each field and the role they play within the Msg struct, making the code easier to understand and maintain.

msg.go

@@ -79,55 +79,68 @@ type PGPType int
 // 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 holds a mapping between AddrHeader keys and their corresponding slices of mail.Address pointers.
 	addrHeader map[AddrHeader][]*mail.Address
 
-	// attachments represent the different attachment File of the Msg
+	// attachments holds a list of File pointers that represent files either as attachments or embeds files in
+	// a Msg.
 	attachments []*File
 
-	// boundary is the MIME content boundary
+	// boundary represents the delimiter for separating parts in a multipart message.
 	boundary string
 
-	// charset represents the charset of the mail (defaults to UTF-8)
+	// charset represents the Charset of the Msg.
+	//
+	// By default we set CharsetUTF8 for a Msg unless overridden by a corresponding MsgOption.
 	charset Charset
 
-	// embeds represent the different embedded File of the Msg
+	// embeds contains a slice of File pointers representing the embedded files in a Msg.
 	embeds []*File
 
-	// encoder represents a mime.WordEncoder from the std lib
+	// encoder is a mime.WordEncoder used to encode strings (such as email headers) using a specified
+	// Encoding.
 	encoder mime.WordEncoder
 
-	// encoding represents the message encoding (the encoder will be a corresponding WordEncoder)
+	// encoding specifies the type of Encoding used for email messages and/or parts.
 	encoding Encoding
 
-	// genHeader is a slice of strings that the different generic mail Header fields
+	// genHeader is a map where the keys are email headers (of type Header) and the values are slices of strings
+	// representing header values.
 	genHeader map[Header][]string
 
-	// isDelivered signals if a message has been delivered or not
+	// isDelivered indicates wether the Msg has been delivered.
 	isDelivered bool
 
-	// middlewares is the list of middlewares to apply to the Msg before sending in FIFO order
+	// middlewares is a slice of Middleware used for modifying or handling messages before they are processed.
+	//
+	// middlewares are processed in FIFO order.
 	middlewares []Middleware
 
-	// mimever represents the MIME version
+	// mimever represents the MIME version used in a Msg.
 	mimever MIMEVersion
 
-	// parts represent the different parts of the Msg
+	// parts is a slice that holds pointers to Part structures, which represent different parts of a Msg.
 	parts []*Part
 
-	// preformHeader is a slice of strings that the different generic mail Header fields
-	// of which content is already preformated and will not be affected by the automatic line
-	// breaks
+	// preformHeader maps Header types to their already preformatted string values.
+	//
+	// Preformatted Header values will not be affected by automatic line breaks.
 	preformHeader map[Header]string
 
 	// pgptype indicates that a message has a PGPType assigned and therefore will generate
-	// different Content-Type settings in the msgWriter
+	// different Content-Type settings in the msgWriter.
 	pgptype PGPType
 
-	// sendError holds the SendError in case a Msg could not be delivered during the Client.Send operation
+	// sendError represents an error encountered during the process of sending a Msg during the
+	// Client.Send operation.
+	//
+	// sendError will hold an error of type SendError.
 	sendError error
 
-	// noDefaultUserAgent indicates whether the default User Agent will be excluded for the Msg when it's sent.
+	// noDefaultUserAgent indicates whether the default User-Agent will be omitted for the Msg when it is
+	// being sent.
+	//
+	// This can be useful in scenarios where headers are conditionally passed based on receipt - i. e. SMTP proxies.
 	noDefaultUserAgent bool
 }