Improve Document#populate documentation, tests

[saveman71]

When migrating to mongoose 6 I found that the new populate syntax for Document#populate was missing some documentation and it was hard to migrate without having a look at the code and guessing that the syntax was the same as Query#populate.

I'm not so familiar with documentation, but I copied the one from Query#populate which seems like an improvement.

Maybe a "sub-page" could describe what arguments Query#populate, Model#populateandDocument#populate` accept in a single place instead of maintaining that in 3 different places, but I'll leave that up to you.

Also added a few tests to demonstrate the arguments I was adding do work.

[saveman71]

Also, for these 3 documentation, I guess it's missing options.populate to do nested population?

[vkarpov15]

@saveman71 that's correct, options.populate is the option for nested populate.