general-concepts/dependencies: mention common pitfalls

[arthurzam]

This commit copies from @mgorny 's blog posts about common pitfalls in writing dependencies, and adds them to the devmanual.

I've added his sign off (based on https://bugs.gentoo.org/587526#c2) but it might have been wrongly done - but I don't really know the correct way to attribute here (I do want to mention him as the super main source).

Closes: https://bugs.gentoo.org/587526