Document the change in semantics from the merge of PR #313.

Author: tzlaine

doc/tutorial.qbk

@@ -1514,9 +1514,10 @@ attribute, simply returns `bool`; this indicates the success or failure of the
 parse.]
 
 [warning _Parser_ assumes that all attributes are semi-regular (see
-`std::semiregular`).  Within the _Parser_ code, attributes are assigned,
-moved, copy, and default constructed.  There is no support for move-only or
-non-default-constructible types.]
+`std::semiregular`).  Move-only types that are nearly `std::semiregular`,
+other than their copy operations, are also supported.  Within the _Parser_
+code, attributes are move assigned, moved, and default constructed.  There is
+no support for non-default-constructible types.]
 
 [heading The attribute type trait, _attr_]