Clarify grub instructions and add note to confirm that the new kernel bo...

Clarifies instructions slightly, and adds some stuff to the menuentry grep to help locate the precise kernel name we want to use. This would be useful on a console without copy/paste functionality, because then you could set that output to a variable and use it in the subsequent sed command.

Additionally, while messing with default kernel settings, I accidentally set the wrong kernel and noticed that GRUB did not display any warning. Instead, it simply booted the first kernel in the list. To avoid this type of mistake going unnoticed, I added a note to confirm that you booted the grsec kernel with `uname -a`.