For one, it's mentioned in Chapter 5: When To Use MongoDB in The Little MongoDB Book. http://openmymind.net/mongodb.pdf
IMO, this is important enough information that it should be mentioned from the start, but it isn't in the tutorial[1], nor can I find it in the FAQ[2].
[1] http://www.mongodb.org/display/DOCS/Tutorial [2] http://docs.mongodb.org/manual/faq/
There's also a reference to the issue in your second link, though it's not super clear. http://docs.mongodb.org/manual/faq/replica-sets/#are-write-o...