bug#20702: 25.0.50; (elisp) `ImageMagick Images': say how to tell if Emacs has IM support, etc.

[Drew Adams]

> > My guess (from trying) would be that (fboundp 'imagemagick-types)
> > is the test that tells you this - in my build it returns nil.
> 
> Actually, M-: (image-type-available-p 'imagemagick) RET and C-h v
> system-configuration-features RET are better ways of getting at that
> information.  The latter should IMO be documented in both manuals.

Very good.  I agree.