Fix bug #18636 with documentation of multi-monitor displays.

 doc/lispref/frames.texi (Multiple Terminals): Improve the description of X
 display names.  Add index entries.
 (Basic Parameters): Add a cross-reference to where X display names
 are described.
 (Position Parameters): Mention that positional parameters of the
 form (+ POS) can be negative if they are on a non-primary monitor
 of a multi-monitor display.
 (Creating Frames): Mention that on multi-monitor displays the
 frame might be positioned differently than specified by the frame
 parameters alist.

 lisp/faces.el (display-grayscale-p): Mention in the doc string that
 the argument can be either a display name or a frame.
 lisp/frame.el (display-pixel-height, display-pixel-width)
 (display-mm-height, display-mm-width, display-backing-store)
 (display-save-under, display-planes, display-color-cells)
 (display-visual-class, display-monitor-attributes-list)
 (display-screens): Mention in the doc string that the argument can
 be either a display name or a frame.  Improve the docs of the
 monitor attributes.

1 files changed, 3 insertions, 1 deletions

diff --git a/lisp/faces.el b/lisp/faces.el
index f316245d165..20665286b4f 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -1814,7 +1814,9 @@ If omitted or nil, that stands for the selected frame's display."
 (declare-function x-display-grayscale-p "xfns.c" (&optional terminal))
 
 (defun display-grayscale-p (&optional display)
-  "Return non-nil if frames on DISPLAY can display shades of gray."
+  "Return non-nil if frames on DISPLAY can display shades of gray.
+DISPLAY should be either a frame or a display name (a string).
+If omitted or nil, that stands for the selected frame's display."
   (let ((frame-type (framep-on-display display)))
     (cond
      ((memq frame-type '(x w32 ns))