Sections 7.b (topic Activating Required Options) of Handbooks contain rather vague recommendations on compiling filesystem support into the kernel. As far as I can judge, only support for the filesystem type used on the root partition needs to be built into the kernel (as opposed to compiling it as a module). Other filesystem drivers may be compiled as modules.
Thanks, committed. I personally still suggest to build in in-kernel but you're correct that it isn't mandatory (so we shouldn't advertise it as such).
