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

[Eli Zaretskii]

> Date: Sat, 10 Nov 2018 14:42:16 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
>  "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.

It is defined by the very next paragraph of the doc string.  I made it
even more clear.

> 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.

This is incorrect when the user specifies just the directory name at
the prompt.  I clarified that only the base name is taken from the
buffer.

Thanks.