In 2009 I was working on building out API docs and looked around for good examples. Stripe was so far ahead of everyone else. I still think they have some of the best documentation out there.
MongoDB's MapReduce implementation is not being deprecated. The primary beneficiary of the V8 implementation is MapReduce, so it should be seen as further investment in this area.
pagesInMemory contains a count of the total number of pages accessed by mongod over the period displayed in overSeconds. The default page size is 4 kilobytes: to convert this value to the amount of data in memory multiply this value by 4 kilobytes.
If your total working set is less than the size of physical memory, over time the value of pagesInMemory will reflect your data size. Conversely, if your data set is greater than the size of the physical RAM, this number will reflect the total size of physical RAM.
Use pagesInMemory in conjunction with overSeconds to help estimate the actual size of the working set.