New PROBLEMS entry (bug#51760)

[Po Lu <luangruo@yahoo.com>]

fcitx doesn't work with xwidgets for some reason beyond my comprehension
(see bug#51760).  IBus and other X input methods work fine, FWIW.

So I'd like to add the following entry to etc/PROBLEMS, under
"Internationalization problems":

** fcitx input methods don't work with xwidgets.

fcitx-based input methods might not work when xwidgets are displayed,
such as inside an xwidget-webkit buffer.  This manifests as the pre-edit
window of the input method disappearing, and the Emacs frame losing
input focus as soon as you try to type anything.  You can work around
this problem by switching to IBus, or by using a native Emacs input
method and disabling XIM altogether.  For example, you can add the
following line:

    Emacs.useXIM: false

In your ~/.Xresources file, then run

    $ xrdb

And restart Emacs.

WDYT about this?  Thanks in advance.