Disallow pass/return of 3-byte struct (#2022), document capability added in #1102.

doc/cc65.sgml

@@ -806,9 +806,8 @@ and the one defined by the ISO standard:
 
 <item>  The datatypes "float" and "double" are not available.
         <p>
-<item>  C Functions may not return structs (or unions), and structs may not
-        be passed as parameters by value. However, struct assignment *is*
-        possible.
+<item>  C Functions may return structs (or unions) by value, but only of
+        1, 2 or 4 byte sizes.
         <p>
 <item>  Most of the C library is available with only the fastcall calling
         convention (<ref id="extension-fastcall" name="see below">).  It means

src/cc65/datatype.c

@@ -803,7 +803,6 @@ const Type* GetStructReplacementType (const Type* SType)
     switch (SizeOf (SType)) {
         case 1:     NewType = type_uchar;   break;
         case 2:     NewType = type_uint;    break;
-        case 3:     /* FALLTHROUGH */
         case 4:     NewType = type_ulong;   break;
         default:    NewType = SType;        break;
     }