Update comments for better clarity and add RFC references

Revised the comments to provide more detailed descriptions and context for each type and constant. Additionally, included relevant RFC document references where applicable to improve understanding of encoding and MIME types.

encoding.go

@@ -4,37 +4,41 @@
 
 package mail
 
-// Charset represents a character set for the encoding
+// Charset is a type wrapper for a string representing different character encodings.
 type Charset string
 
-// ContentType represents a content type for the Msg
+// ContentType is a type wrapper for a string and represents the MIME type of the content being handled.
 type ContentType string
 
-// Encoding represents a MIME encoding scheme like quoted-printable or Base64.
+// Encoding is a type wrapper for a string and represents the type of encoding used for email messages
+// and/or parts.
 type Encoding string
 
-// MIMEVersion represents the MIME version for the mail
+// MIMEVersion is a type wrapper for a string nad represents the MIME version used in email messages.
 type MIMEVersion string
 
-// MIMEType represents the MIME type for the mail
+// MIMEType is a type wrapper for a string and represents the MIME type for the Msg content or parts.
 type MIMEType string
 
-// List of supported encodings
 const (
 	// EncodingB64 represents the Base64 encoding as specified in RFC 2045.
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.8
 	EncodingB64 Encoding = "base64"
 
 	// EncodingQP represents the "quoted-printable" encoding as specified in RFC 2045.
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-6.7
 	EncodingQP Encoding = "quoted-printable"
 
 	// EncodingUSASCII represents encoding with only US-ASCII characters (aka 7Bit)
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-2.7
 	EncodingUSASCII Encoding = "7bit"
 
-	// NoEncoding avoids any character encoding (except of the mail headers)
+	// NoEncoding represents 8-bit encoding for email messages as specified in RFC 6152.
+	// https://datatracker.ietf.org/doc/html/rfc2045#section-2.8
+	// https://datatracker.ietf.org/doc/html/rfc6152
 	NoEncoding Encoding = "8bit"
 )
 
-// List of common charsets
 const (
 	// CharsetUTF7 represents the "UTF-7" charset
 	CharsetUTF7 Charset = "UTF-7"
@@ -133,42 +137,60 @@ const (
 	CharsetGBK Charset = "GBK"
 )
 
-// List of MIME versions
-const (
-	// MIME10 is the MIME Version 1.0
-	MIME10 MIMEVersion = "1.0"
-)
+// MIME10 represents the MIME version "1.0" used in email messages.
+const MIME10 MIMEVersion = "1.0"
 
-// List of common content types
 const (
-	TypeAppOctetStream       ContentType = "application/octet-stream"
+	// TypeAppOctetStream represents the MIME type for arbitrary binary data.
+	TypeAppOctetStream ContentType = "application/octet-stream"
+
+	// TypeMultipartAlternative represents the MIME type for a message body that can contain multiple alternative
+	// formats.
 	TypeMultipartAlternative ContentType = "multipart/alternative"
-	TypeMultipartMixed       ContentType = "multipart/mixed"
-	TypeMultipartRelated     ContentType = "multipart/related"
-	TypePGPSignature         ContentType = "application/pgp-signature"
-	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
-	TypeTextHTML             ContentType = "text/html"
-	TypeTextPlain            ContentType = "text/plain"
+
+	// TypeMultipartMixed represents the MIME type for a multipart message containing different parts.
+	TypeMultipartMixed ContentType = "multipart/mixed"
+
+	// TypeMultipartRelated represents the MIME type for a multipart message where each part is a related file
+	// or resource.
+	TypeMultipartRelated ContentType = "multipart/related"
+
+	// TypePGPSignature represents the MIME type for PGP signed messages.
+	TypePGPSignature ContentType = "application/pgp-signature"
+
+	// TypePGPEncrypted represents the MIME type for PGP encrypted messages.
+	TypePGPEncrypted ContentType = "application/pgp-encrypted"
+
+	// TypeTextHTML represents the MIME type for HTML text content.
+	TypeTextHTML ContentType = "text/html"
+
+	// TypeTextPlain represents the MIME type for plain text content.
+	TypeTextPlain ContentType = "text/plain"
 )
 
-// List of MIMETypes
 const (
+	// MIMEAlternative MIMEType represents a MIME multipart/alternative type, used for emails with multiple versions.
 	MIMEAlternative MIMEType = "alternative"
-	MIMEMixed       MIMEType = "mixed"
-	MIMERelated     MIMEType = "related"
+
+	// MIMEMixed MIMEType represents a MIME multipart/mixed type used for emails containing different types of content.
+	MIMEMixed MIMEType = "mixed"
+
+	// MIMERelated MIMEType represents a MIME multipart/related type, used for emails with related content entities.
+	MIMERelated MIMEType = "related"
 )
 
-// String is a standard method to convert an Charset into a printable format
+// String satisfies the fmt.Stringer interface for the Charset type. It converts a Charset into a printable format.
 func (c Charset) String() string {
 	return string(c)
 }
 
-// String is a standard method to convert an ContentType into a printable format
+// String satisfies the fmt.Stringer interface for the ContentType type. It converts a ContentType into a printable
+// format.
 func (c ContentType) String() string {
 	return string(c)
 }
 
-// String is a standard method to convert an Encoding into a printable format
+// String satisfies the fmt.Stringer interface for the Encoding type. It converts an Encoding into a printable format.
 func (e Encoding) String() string {
 	return string(e)
 }