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.
doc/lispref/frames.texi: (Multiple Terminals): Fix last commit.