docs: improve virsh man page synopses

"optional" is not a very good meta-syntactic construct in our man
page.  I scrubbed this, and additionally improved some documentation
on mutually exclusive options.  For example,

[[--live] [--config] | [--current]]

implies a set of optional flags, where within the set you can have
either --current or a choice of 0, 1, or both --live and --config.

* tools/virsh.pod: Use "[name]" rather than "optional name" for
optional arguments.