bug#15109: 24.3.50; doc of `x-get-selection, `selection-converter-alist', `xselect-convert-to-*'

[Drew Adams]

1. `x-get-selection': Doc string refers you to
`selection-converter-alist', without which you cannot understand
parameter DATA-TYPE.

2. `selection-converter-alist': There is NO doc string.  Please add one
and please make sure it describes (or links to the doc for) the various
`xselect-convert-to-*' functions that one sees in the value of
`selection-converter-alist'.

3. `xselect-convert-to-*' functions, such as
`xselect-convert-to-filename': There is NO doc string.  Please add one
for each such function.


In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-08-07 on ODIEONE
Bzr revision: 113750 lekktu@gmail.com-20130808011911-0jzpc9xuncegg6x9
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 CFLAGS=-O0 -g3 LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'