Touchup "Extending services in Compose"

[charleswhchan]

(The rendered version, if it's easier for review:
https://github.com/charleswhchan/compose/blob/patch-2/docs/extends.md)

[aanand]

Thanks for the contribution!

In my opinion, this guide is going to need to change significantly as a result of #2051, since the process of configuring an app for multiple environments should now be simplified. Perhaps we should incorporate some of these changes when revising it.

[dnephin]

Ya, I'm about to start working on #2141 which will make significant changes to this section.

I'll cherry-pick this commit first and try and preserve some of the edits.

[charleswhchan]

Thanks @dnephin.

[dnephin]

The new version is out. I tried to incorporate some of these changes. It did change pretty drastically, so I wasn't able to apply the cherry-pick, but hopefully the new version is better.

Thanks again for the PR!