Merge from origin/emacs-26

Merge from origin/emacs-26

ff9c962 ; * lisp/ldefs-boot.el: Update.
b4a251c * ; ChangeLog.3 update
d3104e3 * etc/AUTHORS: Update.
a19bfb7 Remove .art from the default list of ImageMagick extensions
6985caa Fix input after setting x-wait-for-event-timeout nil
715388a Fix two warnings in eshell.texi
d49cdd9 * lisp/minibuffer.el (completion-table-dynamic): Improve docs...
5dc4e51 * lisp/emacs-lisp/debug.el (debug-on-variable-change): Doc fi...
76ef805 Fix a typo in ELisp manual
b5e66f4 Update citations of Internet RFCs
57ece2a Fix handling of manpage references divided by hyphenation
7ad0cd6 * doc/misc/eshell.texi: Fix some @ref's.
12b7940 Fix a typo in lispref/syntax.texi

# Conflicts:
#	lisp/gnus/nnrss.el
#	lisp/mail/ietf-drums.el