bug#31730: 26; doc string of `update-glyphless-char-display'

[Drew Adams]

The doc string is not understandable (to me).

1. For starters, neither of the functions parameters is described.
This is the main problem with this doc.

2. And it's not clear what "making the setting" of a variable "take
effect" even means.  Some help in this regard comes from the doc string
of `glyphless-char-display-control', but the statement about making the
setting of the variable take effect could be made more clear.  If the
value is only set effectively by the options's :set function, then just
say, in the doc string of `update-glyphless-char-display', that it is
the :set function for option `glyphless-char-display'.



In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''