Improve the documentation of the XDS support

Improve the documentation of the XDS support

* doc/lispref/frames.texi (Drag and Drop): Rephrase and rearrange
the documentation of XDS support.  Add indexing.  Document
'x-dnd-save-direct' and 'x-dnd-save-direct-immediately'.  Original
patch from Po Lu <luangruo@yahoo.com>.

* lisp/x-dnd.el (x-dnd-types-alist, x-dnd-test-function)
(x-dnd-default-test-function, x-dnd-direct-save-function): Doc
fixes.
(x-dnd-save-direct, x-dnd-save-direct-immediately): Rename the
second argument to FILENAME.  Doc fix.