Move convert-images-externally documentation to Emacs manual

* doc/emacs/files.texi (File Conveniences): Move bit about
convert-images-externally to user manual from lispref manual.

doc/emacs/files.texi

@@ -2161,6 +2161,15 @@ never be rendered using ImageMagick, regardless of the value of
 but Emacs should not).  To disable ImageMagick entirely, change
 @code{imagemagick-types-inhibit} to @code{t}.
 
+@vindex convert-images-externally
+  If Emacs doesn't have native support for the image format in
+question, and @code{convert-images-externally} is non-@code{nil},
+Emacs will try to determine whether there are external utilities that
+can be used to transform the image in question to @acronym{PNG} before
+displaying.  GraphicsMagick, ImageMagick and @command{ffmpeg} are
+currently supported for image conversions.
+
+
 @findex thumbs-mode
 @cindex mode, Thumbs
   The Image-Dired package can also be used to view images as

doc/lispref/display.texi

@@ -6044,14 +6044,6 @@ properties---for example,
 (create-image "foo.xpm" 'xpm nil :heuristic-mask t)
 @end example
 
-@vindex convert-images-externally
-If Emacs doesn't have native support for the image format in question,
-and @code{convert-images-externally} is non-@code{nil}, Emacs will try
-to determine whether there are external utilities that can be used to
-transform the image in question to @acronym{PNG} before displaying.
-GraphicsMagick, ImageMagick and @command{ffmpeg} are currently
-supported for image conversions.
-
 The function returns @code{nil} if images of this type are not
 supported.  Otherwise it returns an image descriptor.
 @end defun