* lisp/textmodes/glyphless-mode.el (glyphless-mode-types): Fix
spelling of "bidirectional".
* lisp/international/characters.el (char-acronym-table): Fix
acronyms for LRI and RLI: use their accepted shorthands.
(glyphless-char-display-control): Fix spelling of doc string.
* doc/lispref/display.texi (Glyphless Chars): Fix spelling and add
a cross-reference.
* doc/lispref/display.texi (Glyphless Chars): Document bidi-control.
* lisp/international/characters.el (char-acronym-table): Add the
ISOLATE characters.
(update-glyphless-char-display): Implement bidi-control, and mark
all Cf characters we don't know about as UNK.
(glyphless--bidi-control-characters): New variable.
(glyphless-char-display-control): Add a new type -- bidi-control.
* lisp/textmodes/glyphless-mode.el (glyphless-mode-types): Add a
new type -- bidi-control.
* doc/emacs/display.texi (Text Display): Mention it.
* doc/lispref/display.texi (Glyphless Chars): Document it.
* lisp/textmodes/glyphless-mode.el: New minor mode (bug#27544).
