[Foundation] Move NSAttributedStringDocumentAttributes.HyphenationFator to shared code.

This property has also been added on macOS (where it didn't exist before), since
both documentation and headers reveal it's available there.

src/Foundation/NSAttributedString.iOS.cs

@@ -41,21 +41,6 @@ namespace Foundation {
 
 	public partial class NSAttributedStringDocumentAttributes : DictionaryContainer {
 #if !MONOMAC && !COREBUILD
-		public float? HyphenationFactor {
-			get {
-				return GetFloatValue (NSAttributedStringDocumentAttributeKey.NSHyphenationFactorDocumentAttribute);
-			}
-			set {
-				if (value is null)
-					RemoveValue (NSAttributedStringDocumentAttributeKey.NSHyphenationFactorDocumentAttribute);
-				else {
-					if (value < 0 || value > 1.0f)
-						throw new ArgumentException ("value must be between 0 and 1");
-					SetNumberValue (NSAttributedStringDocumentAttributeKey.NSHyphenationFactorDocumentAttribute, value);
-				}
-			}
-		}
-
 		public float? DefaultTabInterval {
 			get {
 				return GetFloatValue (NSAttributedStringDocumentAttributeKey.NSDefaultTabIntervalDocumentAttribute);

src/Foundation/NSAttributedStringDocumentAttributes.cs

@@ -42,6 +42,18 @@ namespace Foundation {
 			}
 		}
 
+		public float? HyphenationFactor {
+			get {
+				return GetFloatValue (NSAttributedStringDocumentAttributeKey.NSHyphenationFactorDocumentAttribute);
+			}
+			set {
+				if (value < 0 || value > 1.0f)
+					throw new ArgumentOutOfRangeException (nameof (value), value, "Value must be between 0 and 1");
+
+				SetNumberValue (NSAttributedStringDocumentAttributeKey.NSHyphenationFactorDocumentAttribute, value);
+			}
+		}
+
 		public NSStringEncoding? StringEncoding {
 			get {
 				return (NSStringEncoding?) (long?) GetNIntValue (NSAttributedStringDocumentAttributeKey.NSCharacterEncodingDocumentAttribute);