bug#33339: 26; doc string of `write-file`

[Drew Adams]

 "If you specify just a directory name as FILENAME, that means to use
  the default file name but in that directory.  You can also yank
  the default file name into the minibuffer to edit it, using M-n."

"The default file name" is undefined.  How is a user to know what we
mean?  It turns out that in this particular case what we mean is this:

(file-name-nondirectory (or buffer-file-name (buffer-name)))

Not obvious from the description.

To start with, the doc string should say that it prompts for a file
name.  If the current buffer is visiting a file then that file's name is
the default file name for reading.  Otherwise the buffer name is the
default file name for reading.

The last part could be more exact.  The default name in that case is
actually this:

 (expand-file-name (file-name-nondirectory (buffer-name))
                   default-directory) 

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''