Add docs about strict separation of parameters
Add docs about strict separation of parameters

This addresses a question raised in #161 and is also kind-of related to #108.

The new section could also have been added to /parameters/, but i feel
it is more relevant here for users who are starting to learn about
subcommands. The behavior is totally irrelevant when writing a program
without subcommands, however sophisticated the usage of options and
arguments might be.

GeoffColburn/click,TomRegan/click,cbandera/click,oss6/click,dastergon/click,gambogi/click,scalp42/click,MakerDAO/click,pgkelley4/click,amjith/click,Nudies/click,ma-ric/click,hugopeixoto/click,mitsuhiko/click,Akasurde/click,her0e1c1/click,pallets/click,naoyat/click,glorizen/click,jvrsantacruz/click,pombredanne/click,andela-ooladayo/click,RonnyPfannschmidt/click,khwilson/click,hackebrot/click,willingc/click,hellodk/click,ionelmc/click,ternus/click,lucius-feng/click,polinom/click,nchammas/click