bootparam.7: Describe 'rootflags' and 'rootdelay' kernel parameters

Patch based on text from Documentation/kernel-parameters.txt

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
bootparam.7: Minor tweaks to Petr Gajdos's patch

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>