The docs currently state

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.

(http://docs.mongodb.org/manual/reference/server-status/#workingset)

However, the pagesInMemory value can grow to be much larger than the actual memory size of the system. If your dataset is greater than the size of physical ram, pagesInMemory will grow until it either reaches the size of the working set, or the size the "Rolling" structure contains before it rolls over, which can vary depending on usage. I suggest either removing or rewriting the second sentence of this paragraph.