mu4e: mention privacy aspects of browser

Mention the privacy aspects of opening a message in a browser, and what
to do about it.

mu4e/mu4e-actions.el

@@ -87,13 +87,15 @@ return the filename."
 (defun mu4e-action-view-in-browser (msg)
   "View the body of the message in a browser.
 You can influence the browser to use with the variable
-`browse-url-generic-program'."
+`browse-url-generic-program', and see the discussion of privacy
+aspects in `(mu4e) Displaying rich-text messages'."
   (browse-url (concat "file://"
 		(mu4e~write-body-to-html msg)))) 
 
 (defun mu4e-action-view-with-xwidget (msg)
   "View the body of the message inside xwidget-webkit. This is
-only available in emacs 25+."
+only available in emacs 25+; also see the discussion of privacy
+aspects in `(mu4e) Displaying rich-text messages'."
   (unless (fboundp 'xwidget-webkit-browse-url)
     (mu4e-error "No xwidget support available"))
   (xwidget-webkit-browse-url

mu4e/mu4e.texi

@@ -1319,7 +1319,6 @@ If your emacs does not have @t{shr} yet, it can be useful to use a
 custom method. For that, you can set the variable
 @code{mu4e-html2text-command} to either a shell command or a function
 instead.
- 
 
 @subsection Html2text commands
 
@@ -1359,6 +1358,17 @@ If @code{mu4e-html2text-command} refers to an elisp function, it is
 expected to take the current buffer in html as input, and transform it
 into text (just like the @code{html2text} function).
 
+@subsection Privacy aspects
+@anchor{Privacy aspects}
+
+When opening your messages in a graphical browser, it may expose you
+doing so to the sender, due to the presence of specially crafted image
+URLs, or Javascript.
+
+If that is an issue, it is recommended to use a browser (or browser
+profile) that does not load images. The same applies to Javascript.
+
+
 @node MSGV Crypto
 @section Crypto
 
@@ -3693,7 +3703,7 @@ defined for this. Simply add to your configuration:
 @end lisp
 Now, when viewing such a difficult message, type @kbd{aV}, and the message
 opens inside a webbrowser. You can influence the browser with
-@code{browse-url-generic-program}.
+@code{browse-url-generic-program}; and see @ref{Privacy aspects}.
 @item @emph{How can read encrypted messages that I sent?}. Since you do not own the
 recipient's key you typically cannot read those mails - so the trick is
 to encrypt outgoing mails with your key, too. This can be automated by