Add GRUB 2 restoration steps to help manual (#734376)

Also add short description of GRUB 2 and GRUB Legacy, and list of
GNU/Linux distributions and which version started using GRUB 2.

Bug #734376 - Add GRUB 2 Restore Steps to GParted Manual
Add GRUB Legacy live media boot step to help manual (#734376)

Bug #734376 - Add GRUB 2 Restore Steps to GParted Manual
Remove duplication in fixing GRUB boot section of help manual (#734376)

Part of the list of actions that could cause a computer to fail to
boot were duplicated.  Remove this duplication from the fixing GRUB
boot problems section of the help manual.

Bug #734376 - Add GRUB 2 Restore Steps to GParted Manual
Improve wording of fixing OS boot problem section in help manual (#734376)

Place more emphasis on the instructions to restore GRUB, and less on
other boot loaders.

Bug 734376 - Add GRUB 2 Restore Steps to GParted Manual
Add GParted Live USB flash drive tip to help manual (#734376)

Add tip that the GParted Live image can be written to a USB flash
drive to be used in computers that can boot from USB.

Bug 734376 - Add GRUB 2 Restore Steps to GParted Manual