Improve "Testing Documentation" in contributing guide

[wouterj]

We should improve http://symfony.com/doc/current/contributing/documentation/format.html#testing-documentation to make it a bit less confusing.

From #3917:

• We should explain how the build will look like.
• Tell about the additional _ext directory and how not to add it to the PR.

[snoek09]

The _exts directory is already in the gitignore file.

[stof]

@snoek09 the _exts directory is a submodule

[snoek09]

Also mentioned here: #3917 (comment)
Seems to me it's not needed any more to mention this.

[wouterj]

I think we should tell something about the submodule stuff, as it can introduce weird results for git status when you're on an out of date branch. We should explain how to avoid/remove this troube imo

[xabbuh]

I am not sure if we really need to document this anymore now that you can see how your PR will look like thanks to our platform.sh previews.

[javiereguiluz]

I agree with @xabbuh. The current situation is much better than when this issue was created. Now we validate the syntax with Travis and we provide a real doc rendering thanks to Platform.sh.