Merge #11555 as_string doc fix from 3.1.

Doc/library/email.message.rst

@@ -46,15 +46,16 @@ Here are the methods of the :class:`Message` class:
       be generated or modified).
 
       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 mangles
-      lines that begin with ``From``.  For more flexibility, instantiate a
+      format the message the way you want.  For example, by default it does
+      not do the mangling of lines that begin with ``From`` that is
+      required by the unix mbox format.  For more flexibility, instantiate a
       :class:`~email.generator.Generator` instance and use its :meth:`flatten`
       method directly.  For example::
 
          from io import StringIO
          from email.generator import Generator
          fp = StringIO()
-         g = Generator(fp, mangle_from_=False, maxheaderlen=60)
+         g = Generator(fp, mangle_from_=True, maxheaderlen=60)
          g.flatten(msg)
          text = fp.getvalue()
 

Lib/email/message.py

@@ -157,8 +157,7 @@ class Message:
         header.
 
         This is a convenience method and may not generate the message exactly
-        as you intend because by default it mangles lines that begin with
-        "From ".  For more flexibility, use the flatten() method of a
+        as you intend.  For more flexibility, use the flatten() method of a
         Generator instance.
         """
         from email.generator import Generator