This is another "extra" patch in my series to improve the organization 
of invoke.texi and make it easier for users to find things.

It seemed to me that "Hardware Models and Configurations" was not an 
obvious name for the section on machine-dependent options.  The "Option 
Summary" section already used "Machine-Dependent Options" (minus the 
hyphen) to describe this group of options, so I went with that instead.

I also thought that using 30+-year-old processor names like "68010 vs 
68020" as an example seemed pretty archaic, so I also rewrote the @menu 
description and introductory blurb for the section to be more generic 
and better reflect that nowadays target-specific options do a lot more 
than just let you choose the hardware model to compile for.

I've committed this patch.

-Sandra