Clarify if `@multimethod` works with position-or-keyword arguments

[ParadaCarleton]

Right now @multimethod dispatch is documented as "positional only"--does that mean it only dispatches on arguments that are positional-only, or does it dispatch on arguments that are positional-or-keyword?

[coady]

multimethod only dispatches on positional arguments (from the caller). Which means positional-only or positional-or-keyword parameters (the function definition).

multidispatch includes support for keyword arguments.