Revise argument terminology?

[shadowspawn]

While working on #1315 and #1332 I did a rethink of the terminology for talking about arguments, expecially options and option-arguments. Pretty happy with how it worked out in that content, and considering rewriting README to match.

Some background research is in #1332 (comment)

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Terminology

The command line arguments are made up of options, option-arguments, commands, and command-arguments.

Term	Explanation
option	an argument which is a - followed by a character, or -- followed by a word (or hyphenated words), like -s or --short
option-argument	some options can take an argument
command	a program or command can have subcommands
command-argument	argument for the command (and not an option or option-argument)

For example:

my-utility command --option option-argument command-argument-1 command-argument-2

In other references options are sometimes called flags, and command-arguments are sometimes called positional arguments or operands.

[shadowspawn]

Released in Commander v6.2.0