Adding Documentation for required shop query parameter

[jpotts18]

It's yours now. Do what you want with it.

[dylanahsmith]

Manually merged as commit e738f0d1390c400c1b9311be94ede45b27d256a4 then edited with commit d6926463399166adc84ccbf582e27b62f299a0b1 to have this be part of the usage with a example form to illustrate how to use it.

Thanks for bringing these problems with the documentation to our attention