document envvars in BPMNs

[MattDodsonEnglish]

• Document how to use them.
• Add where they can be used in the element reference
• Include links or explanation to a process variable explanation

They should work in:
GQL

• timeouts
• maxpayload
• URL
• query

JsonSchema

• schema
  Datasource Method Call
• Arguments

REST

• Timeout
• maxpayload
• URL

CA

• Body (if string, []string or map[string]string)
• Headers and their values
• URL params and their content

Intermediate Throw

• Stream
• Payload