Refactor MsgOption comments for clarity

Revised the comments for MsgOption functions to provide clearer explanations of their purpose and usage. Added detailed descriptions for options such as WithMIMEVersion and WithBoundary to clarify their contexts and constraints.

msg.go

@@ -144,13 +144,14 @@ type Msg struct {
 	noDefaultUserAgent bool
 }
 
-// SendmailPath is the default system path to the sendmail binary
+// SendmailPath is the default system path to the sendmail binary - at least on standard Unix-like OS.
 const SendmailPath = "/usr/sbin/sendmail"
 
-// MsgOption returns a function that can be used for grouping Msg options
+// MsgOption is a function type that modifies a Msg instance during its creation or initialization.
 type MsgOption func(*Msg)
 
-// NewMsg returns a new Msg pointer
+// NewMsg creates a new email message with optional MsgOption functions that customize various aspects of the
+// message.
 func NewMsg(opts ...MsgOption) *Msg {
 	msg := &Msg{
 		addrHeader:    make(map[AddrHeader][]*mail.Address),
@@ -161,7 +162,7 @@ func NewMsg(opts ...MsgOption) *Msg {
 		mimever:       MIME10,
 	}
 
-	// Override defaults with optionally provided MsgOption functions
+	// Override defaults with optionally provided MsgOption functions.
 	for _, option := range opts {
 		if option == nil {
 			continue
@@ -175,49 +176,63 @@ func NewMsg(opts ...MsgOption) *Msg {
 	return msg
 }
 
-// WithCharset overrides the default message charset
+// WithCharset sets the Charset type for a Msg during its creation or initialization.
 func WithCharset(c Charset) MsgOption {
 	return func(m *Msg) {
 		m.charset = c
 	}
 }
 
-// WithEncoding overrides the default message encoding
+// WithEncoding sets the Encoding type for a Msg during its creation or initialization.
 func WithEncoding(e Encoding) MsgOption {
 	return func(m *Msg) {
 		m.encoding = e
 	}
 }
 
-// WithMIMEVersion overrides the default MIME version
+// WithMIMEVersion sets the MIMEVersion type for a Msg during its creation or initialization.
+//
+// Note that in the context of email, MIME Version 1.0 is the only officially standardized and supported
+// version. While MIME has been updated and extended over time (via various RFCs), these updates and extensions
+// do not introduce new MIME versions; they refine or add features within the framework of MIME 1.0.
+// Therefore there should be no reason to ever use this MsgOption.
+// https://datatracker.ietf.org/doc/html/rfc1521
+// https://datatracker.ietf.org/doc/html/rfc2045
+// https://datatracker.ietf.org/doc/html/rfc2049
 func WithMIMEVersion(mv MIMEVersion) MsgOption {
 	return func(m *Msg) {
 		m.mimever = mv
 	}
 }
 
-// WithBoundary overrides the default MIME boundary
+// WithBoundary sets the boundary of a Msg to the provided string value during its creation or initialization.
+//
+// Note that by default we create random MIME boundaries. This should only be used if a specific boundary is
+// required.
 func WithBoundary(b string) MsgOption {
 	return func(m *Msg) {
 		m.boundary = b
 	}
 }
 
-// WithMiddleware add the given middleware in the end of the list of the client middlewares
+// WithMiddleware adds the given Middleware to the end of the list of the Client middlewares slice. Middleware
+// are processed in FIFO order.
 func WithMiddleware(mw Middleware) MsgOption {
 	return func(m *Msg) {
 		m.middlewares = append(m.middlewares, mw)
 	}
 }
 
-// WithPGPType overrides the default PGPType of the message
+// WithPGPType sets the PGP type for the Msg during its creation or initialization, determining the encryption or
+// signature method.
 func WithPGPType(pt PGPType) MsgOption {
 	return func(m *Msg) {
 		m.pgptype = pt
 	}
 }
 
-// WithNoDefaultUserAgent configures the Msg to not use the default User Agent
+// WithNoDefaultUserAgent disables the inclusion of a default User-Agent header in the Msg during its creation or
+// initialization.
 func WithNoDefaultUserAgent() MsgOption {
 	return func(m *Msg) {
 		m.noDefaultUserAgent = true