Updates for Display chapter of Lisp manual.

Updates for Display chapter of Lisp manual.

* doc/lispref/display.texi (Image Formats): Add imagemagick type.
(Image Descriptors): Mention how they are used.
(ImageMagick Images): Clarify role of imagemagick-register-types.
(Character Display): Don't mention glyph tables.
(Display Tables): Use make-glyph-code in example.
(Glyphs): Avoid "simple glyph code" terminology.  Note that glyph
tables are semi-obsolete.  De-document create-glyph.
(Glyphless Chars): Note that display tables override this.
(Bidirectional Display): Copyedits.  Introduce "bidirectional
reordering" terminology, and use it.

* doc/emacs/files.texi (File Conveniences): Clarify Imagemagick discussion.

* lisp/image.el (imagemagick-types-inhibit)
(imagemagick-register-types): Doc fix.