doc: Fix for too many brackets in command synopses on man pages

The default for the choice attribute of the <arg> element is "opt",
which would normally put the argument inside brackets.  But the DSSSL
stylesheets contain a hack that treats <arg> directly inside <group>
specially, so that <group><arg>-x</arg><arg>-y</arg></group> comes out
as [ -x | -y ] rather than [ [-x] | [-y] ], which it would technically
be.  But when building man pages, this doesn't work, and so the
command synopses on the man pages contain lots of extra brackets.

By putting choice="opt" or choice="plain" explicitly on every <arg>
and <group> element, we avoid any toolchain dependencies like that,
and it also makes it clearer in the source code what is meant.

In passing, make some small corrections in the documentation about
which arguments are really optional or not.