bug#11334: 24.1.50; doc of `dired-do-copy'

[Drew Adams]

This is a user command, so the doc is more important than for an
internal function.

What on earth does this mean (from the doc string)?

 "This normally preserves the last-modified date when copying."

What does "normally" mean?  When does it not preserve that date+time?
How can a user control what date+time is used when the file is copied?

A little help for users, please.  See the Emacs manual for inspiration.
The doc string here should refer to `dired-copy-preserve-time', and it
should drop the vague "normally" hand-waving.

In GNU Emacs 24.1.50.1 (i386-mingw-nt5.1.2600)
 of 2012-04-23 on MARVIN
Bzr revision: 108006
agustin.martin@hispalinux.es-20120423103325-xmra3329elgzhmpc
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
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'