Add some clarifications about the mapping interface presented by
rfc822.Message objects, based on comments from Barry.
diff --git a/Doc/lib/librfc822.tex b/Doc/lib/librfc822.tex
index 2f68bb8..56f6c74 100644
--- a/Doc/lib/librfc822.tex
+++ b/Doc/lib/librfc822.tex
@@ -211,13 +211,15 @@
\code{None}.
\end{methoddesc}
-\class{Message} instances also support a read-only mapping interface.
+\class{Message} instances also support a limited mapping interface.
In particular: \code{\var{m}[name]} is like
\code{\var{m}.getheader(name)} but raises \exception{KeyError} if
there is no matching header; and \code{len(\var{m})},
\code{\var{m}.has_key(name)}, \code{\var{m}.keys()},
\code{\var{m}.values()} and \code{\var{m}.items()} act as expected
-(and consistently).
+(and consistently). \class{Message} instances also support the
+mapping writable interface \code{\var{m}[name] = value} and \code{del
+\var{m}[name]}.
Finally, \class{Message} instances have two public instance variables: