User's Guide: TAP setup tweakage

Highlight that the "-expected-id" probably comes from vendor
documentation, and that it *should* be used where possible.

Don't use ircapture/irmask in examples, to help discourage
use of those params when they're not required.  Explain a
bit better about why/when those params get used.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>