bug#11654: 24.1; doc of `icomplete-with-completion-tables' and `icomplete-simple-completing-p'

[Drew Adams]

1. The doc string of `icomplete-with-completion-tables' should say what
a value of `t' means.  It has a special meaning.
 
2. The doc string of `icomplete-simple-completing-p' should be corrected
wrt its "Conditions" description.  What's missing is the logic that uses
`icomplete-with-completion-tables'.
 
3. The value of `icomplete-with-completion-tables' is not changed in
`icomplete.el'.  It is changed only in `window.el'.  Presumably this var
exists for libraries (presumably including 3rd-party code) to bind or
set.  All the more reason for it to be better documented.

In GNU Emacs 24.1.1 (i386-mingw-nt5.1.2600)
 of 2012-06-01 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --no-opt --enable-checking --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include'