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

...d.
I found in the Python style guide where it says to use 3 space indent for reST, but I'd prefer to stick with 4 space indents. IMO, it's confusing to mix 3 and 4 space indent (e.g. code examples) and our docs mostly use 4 I think (at least, I've converted some places that use 3 to 4). Do you know why Python suggests to use 3 spaces? Should we update our "writing documentation" guide with this guideline?