Doc tweaks

* doc/lispref/frames.texi (Multiple Terminals): Copyedits.

* lisp/frame.el (display-monitor-attributes-list): Doc tweaks.

* nt/README.W32, src/ChangeLog.10, src/w32term.c:
Standardize on "taskbar" rather than "task bar", since that is what
most references seem to use; e.g.
http://en.wikipedia.org/wiki/Taskbar
http://windows.microsoft.com/en-us/windows7/products/features/windows-taskbar
* doc/lispref/frames.texi: Tweak previous tweaks.