Don't confuse how Texinfo outputs @var with the input

Don't confuse how Texinfo outputs @var with the input

* doc/emacs/rmail.texi (Movemail):
* doc/lispref/control.texi (Pattern matching case statement):
* doc/lispref/frames.texi (Size and Position):
* doc/lispref/processes.texi (Asynchronous Processes):
* doc/lispref/text.texi (Document Object Model):
* doc/lispref/windows.texi (Coordinates and Windows):
Do not upper-case the argument of @var.