bug#27996: open-dribble-file docstring improvement

[積丹尼 Dan Jacobson]

open-dribble-file is an interactive built-in function in ‘C source code’.

> Start writing all keyboard characters to a dribble file called FILE.
> If FILE is nil, close any open dribble file.
> The file will be closed when Emacs exits.

On the last line I would say

< Else the file will be closed when Emacs exits.

as some readers wouldn't notice the second line as that is just talking
about parameters.

Also mention how often it is written. Every keystroke? Every newline?
Also when one does C-x C-s etc.