docs; fix incorrect example
Update MongoDB dependency to 1.3.17
Fixed issue with positional operator on ref'd docs

Positional operator was throwing an error when you tried to update a
referenced document on another doc. fixes #1572
simplify test for 1572, ocd
Limit applies to individual items on population

fixes #1490, the limit is now respected on an item by item basis when
populating an array of items.
refactor 1613 test
release 3.6.16
now working on 3.6.17
Merge branch '3.6.x' into gh-pages
website; regen for 3.6.16