doc/kernel-parameters.txt: clarify clk_ignore_unused

Refine the definition around clk_ignore_unused, which caused some
confusion recently on the linux-fbdev and linux-arm-kernel mailing
lists[0].

[0] http://lkml.kernel.org/r/<20140929135358.GC30998@ulmo

>

Signed-off-by: Mike Turquette <mturquette@linaro.org>