Fixed #18731 -- Added an example about customizing "makemessages" comman...

...d.
> Do you know why Python suggests to use 3 spaces?

Probably the reason is that indentation is significant in reST and the minimum length of an indentation should be 3(e.g. the `.. foo::` directives):

```rst
.. function:: option_name
   Description of the function.
```

> Should we update our "writing documentation" guide with this guideline?

I don't think we follow that suggestion consistently in the Python documentation :) 

I've converted this to use 4 space indentation.