Add documentation about as_string() mangling From_ lines.
diff --git a/Doc/lib/emailmessage.tex b/Doc/lib/emailmessage.tex
index f732054..54e5cac 100644
--- a/Doc/lib/emailmessage.tex
+++ b/Doc/lib/emailmessage.tex
@@ -37,9 +37,10 @@
\var{unixfrom} is \code{True}, the envelope header is included in the
returned string. \var{unixfrom} defaults to \code{False}.
-Note that this method is provided as a convenience and may not always
-format the message the way you want. For more flexibility,
-instantiate a \class{Generator} instance and use its
+Note that this method is provided as a convenience and may not always format
+the message the way you want. For example, by default it mangels lines that
+begin with \code{From }. For more flexibility, instantiate a
+\class{Generator} instance and use its
\method{flatten()} method directly. For example:
\begin{verbatim}