Refactor documentation for Importance methods

Updated the documentation for NumString, XPrioString, and String methods in the Importance type to provide clearer descriptions of their behavior and return values. Enhanced comments for better readability and maintainability of the code.

header.go

@@ -136,8 +136,14 @@ const (
 	ImportanceUrgent
 )
 
-// NumString returns a numerical string representation of the Importance, mapping ImportanceHigh and
-// ImportanceUrgent to "1" and others to "0".
+// NumString returns a numerical string representation of the Importance level.
+//
+// This method maps ImportanceHigh and ImportanceUrgent to "1", while ImportanceNonUrgent and ImportanceLow
+// are mapped to "0". Other values return an empty string.
+//
+// Returns:
+//   - A string representing the numerical value of the Importance level ("1" or "0"), or an empty string
+//     if the Importance level is unrecognized.
 func (i Importance) NumString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -153,8 +159,14 @@ func (i Importance) NumString() string {
 	}
 }
 
-// XPrioString returns the X-Priority string representation of the Importance, mapping ImportanceHigh and
-// ImportanceUrgent to "1" and others to "5".
+// XPrioString returns the X-Priority string representation of the Importance level.
+//
+// This method maps ImportanceHigh and ImportanceUrgent to "1", while ImportanceNonUrgent and ImportanceLow
+// are mapped to "5". Other values return an empty string.
+//
+// Returns:
+//   - A string representing the X-Priority value of the Importance level ("1" or "5"), or an empty string
+//     if the Importance level is unrecognized.
 func (i Importance) XPrioString() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -170,8 +182,14 @@ func (i Importance) XPrioString() string {
 	}
 }
 
-// String satisfies the fmt.Stringer interface for the Importance type and returns the string representation of the
-// Importance level.
+// String satisfies the fmt.Stringer interface for the Importance type and returns the string
+// representation of the Importance level.
+//
+// This method provides a human-readable string for each Importance level.
+//
+// Returns:
+//   - A string representing the Importance level ("non-urgent", "low", "high", or "urgent"), or an empty
+//     string if the Importance level is unrecognized.
 func (i Importance) String() string {
 	switch i {
 	case ImportanceNonUrgent:
@@ -187,13 +205,20 @@ func (i Importance) String() string {
 	}
 }
 
-// String satisfies the fmt.Stringer interface for the Header type and returns the string representation of the Header.
+// String satisfies the fmt.Stringer interface for the Header type and returns the string
+// representation of the Header.
+//
+// Returns:
+//   - A string representing the Header.
 func (h Header) String() string {
 	return string(h)
 }
 
-// String satisfies the fmt.Stringer interface for the AddrHeader type and returns the string representation of the
-// AddrHeader.
+// String satisfies the fmt.Stringer interface for the AddrHeader type and returns the string
+// representation of the AddrHeader.
+//
+// Returns:
+//   - A string representing the AddrHeader.
 func (a AddrHeader) String() string {
 	return string(a)
 }