document canonical name and move vars down to usage
document canonical name and move vars down to usage

Closes Homebrew/homebrew#20719.

aw1621107/brew,gregory-nisbet/brew,claui/brew,zmwangx/brew,alyssais/brew,EricFromCanada/brew,tonyg/homebrew,pseudocody/brew,rwhogg/brew,Linuxbrew/brew,aw1621107/brew,amar-laksh/brew_sudo,AnastasiaSulyagina/brew,mahori/brew,claui/brew,palxex/brew,EricFromCanada/brew,JCount/brew,claui/brew,reitermarkus/brew,jmsundar/brew,konqui/brew,JCount/brew,maxim-belkin/brew,mgrimes/brew,gregory-nisbet/brew,nandub/brew,MikeMcQuaid/brew,mistydemeo/homebrew,bfontaine/brew,palxex/brew,jmsundar/brew,MikeMcQuaid/brew,JCount/brew,rwhogg/brew,nandub/brew,reitermarkus/brew,mgrimes/brew,reitermarkus/brew,bfontaine/brew,tonyg/homebrew,alyssais/brew,MikeMcQuaid/brew,muellermartin/dist,DomT4/brew,claui/brew,DomT4/brew,mahori/brew,mgrimes/brew,reelsense/brew,pseudocody/brew,muellermartin/dist,reelsense/brew,maxim-belkin/brew,nandub/brew,amar-laksh/brew_sudo,vitorgalvao/brew,hanxue/linuxbrew,DomT4/brew,gordonmcshane/homebrew,zmwangx/brew,MikeMcQuaid/brew,toonetown/homebrew,AnastasiaSulyagina/brew,muellermartin/dist,nandub/brew,Homebrew/brew,Linuxbrew/brew,staticfloat/brew,Homebrew/brew,Homebrew/brew,konqui/brew,JCount/brew,konqui/brew,hanxue/linuxbrew,sjackman/homebrew,toonetown/homebrew,gregory-nisbet/brew,toonetown/homebrew,vitorgalvao/brew,pseudocody/brew,sjackman/homebrew,staticfloat/brew,DomT4/brew,Linuxbrew/brew,vitorgalvao/brew,mistydemeo/homebrew,aw1621107/brew,mahori/brew,EricFromCanada/brew,gordonmcshane/homebrew,bfontaine/brew,amar-laksh/brew_sudo,reelsense/brew,hanxue/linuxbrew,Homebrew/brew,sjackman/homebrew,jmsundar/brew,staticfloat/brew,reitermarkus/brew,sjackman/homebrew,ilovezfs/brew,AnastasiaSulyagina/brew,EricFromCanada/brew,maxim-belkin/brew,tonyg/homebrew,konqui/brew,zmwangx/brew,palxex/brew,rwhogg/brew,ilovezfs/brew,mistydemeo/homebrew,vitorgalvao/brew,Linuxbrew/brew,ilovezfs/brew,gordonmcshane/homebrew,mahori/brew,alyssais/brew