docs: admin-guide: Clarify sentences

Changes to make the text more formal and organized. The reasons are now cited and described at the same time.
Minor grammatical problems have also been fixed.

Signed-off-by: Cristian Souza <cristianmsbr@gmail.com>
Link: https://lore.kernel.org/r/20200411010201.GA22706@darkstar


Signed-off-by: Jonathan Corbet <corbet@lwn.net>