Add some information about finding docs with undefined values

[jstell]

In case someone else runs into the puzzling situation where the shell shows a null value, but querying for null doesn't work.

[tychoish]

Thanks for these pull requests. Before I merge them in, could you please sign the MongoDB/10gen Contributors agreement? It's located here: http://www.10gen.com/legal/contributor-agreement

Thanks!

[kay-kim]

Hi Jason --
thank you for the pull request and as with your other pull request, apologies for the tardiness of the response. I have merged your other pull request in. As for this particular change, because undefined is deprecated, we are keeping the documentation minimal. As such, I will not be merging in this pull request. But thanks ever so much.

FYI -- for your $in query, that query, I believe, will return both docs with undefined values and those with missing fields.

As always, warmest regards and we really appreciate your time and involvement.

Kay