Update to r42938:

This patch does the following:

makes 4.2 clearer and tightens some of the headings
removed reference to learn more about single-user mode as it didn't say anything more; instead, summarized single-user mode here
made intro to permissions clearer, the next patch will work on the rest of this section

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D4752