Update WITHOUT_KERNEL_SYMBOLS description

We have installed kernel debug data under /usr/lib/debug/ for some time
now, so the suggestion to set WITHOUT_KERNEL_SYMBOLS for small root
partitions is no longer valid.

Also call them "debug symbol files" rather than just "symbol files",
since they contain much more than just symbols. The kernel also
includes (some) symbols, regardless of the setting of this knob.

MFC after: 1 week