Manual pages: runuser.1, su.1: miscellaneous wording and formatting fixes

Manual pages: runuser.1, su.1: miscellaneous wording and formatting fixes

Various wording and formatting fixes. Nothing too contentnious, I think,
so I rolled these changes into one patch.

Since there is much common text in su.1 and runuser.1, I've combined
the changes to both pages into one patch, and, as far as possible,
ensured that changes to the common pieces of text match across the
two pages.

Signed-off-by: Michael Kerrisk (man-pages) <mtk.manpages@gmail.com>