Update /proc/[number]/cmdline description.

Update /proc/[number]/cmdline description.
It used to be true that the command line arguments were
not accessible when the process had been swapped out.
In ancient kernels (circa 2.0.*) the problem was that the
kernel relied on get_phys_addr to access the user space buffer,
which stopped working as soon as the process was swapped out.
Recent kernels use get_user_pages for the same purpose and thus
they should not have that limitation.