docs: align nsenter.1 man page with howto-man-page.txt

docs: align nsenter.1 man page with howto-man-page.txt

Most visible change; the --target option has a path - explanation table
instead a long paragraph.  This makes pairing of the information easier
for an average user such as me.

The rest of the changes are about aligning with howto, i.e., fix spaces
after dots, URL & mail address macros, remove inline emphasis where
possible, mark directory paths and words with underscore to be line
breakable, use optional option syntax from howto, cut line lenght to 80
chars.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>