Merge from origin/emacs-26

5afbf62 Fix emacsclient check for term.el buffer (Bug#21041)
5132a58 Improve documentation of 'set-fontset-font'
cd90325 Improve documentation of M-?
155a885 Reinterpret Esperanto characters in iso-transl as iso-8859-3.
a0ef733 Fix Flyspell mode when several languages are mixed in a buffer

diff --cc doc/emacs/maintaining.texi
Simple merge

diff --cc doc/lispref/display.texi
Simple merge

diff --cc lib-src/emacsclient.c
Simple merge

diff --cc lisp/progmodes/xref.el
Simple merge

diff --cc lisp/textmodes/flyspell.el
Simple merge