Clarify documentation re switch expecting an argument but none is provided.

When a switch is expecting an argument but fails to be provided with one, the
value of the corresponding element in the options hash is set to the undefined
value.  The documentation did not make this clear.

The documentation for getopts() and getopt() has been revised and additional
tests have been provided to demonstrate the point.  Tweaked in response to
feedback from Karl Williamson++.

Also, unit tests found in lib/Getopt/Std.t actually for Getopt::Long have been
removed.  A patch holding those tests has been submitted to Getopt-Long's bug
queue on rt.cpan.org.

For: RT #41359