emacs: drop setting mail-user-agent, and document how to set it

After some discussion [1], I decided it is better to make notmuch users
who rely on this behaviour customize mail-user-agent. This is
consistent with the behaviour of other emacs mail packages.

[1]: id:87k0nuhfrk.fsf@toryanderson.com

NEWS

@@ -1,6 +1,13 @@
 Notmuch 0.33 (UNRELEASED)
 =========================
 
+Emacs
+-----
+
+`notmuch` no longer sets `mail-user-agent` on load. To restore the
+previous behaviour of using notmuch to send mail by default, customize
+`mail-user-agent` to `notmuch-user-agent`.
+
 Vim
 ---
 

doc/notmuch-emacs.rst

@@ -366,6 +366,16 @@ Importing Mail
 :index:`notmuch-poll-script`
    |docstring::notmuch-poll-script|
 
+Sending Mail
+------------
+
+:index:`mail-user-agent`
+
+       Emacs consults the variable :ref:`mail-user-agent` to choose a mail
+       sending package for commands like :ref:`report-emacs-bug` and
+       :ref:`compose-mail`.  To use ``notmuch`` for this, customize this
+       variable to the symbol :ref:`notmuch-user-agent`.
+
 Init File
 ---------
 

emacs/notmuch.el

@@ -1162,8 +1162,6 @@ Point should be at the beginning of the line."
 
 ;;; _
 
-(setq mail-user-agent 'notmuch-user-agent)
-
 (provide 'notmuch)
 
 ;; After provide to avoid loops if notmuch was require'd via notmuch-init-file.