<documentation> Add documentation for ENV: and ENV:* cartridge event subscriptions

Both the ENV:* wildcard event subscription type and the ENV: type
which preceded it were undocumented. Not anymore!
Further event type doc tweaks

Switched order of wildcard/non-wildcard entries

Moved info pertaining to both wildcard and non-wildcard entries to top
of section, placed wc/non-wc info in subsections
Merge pull request #3570 from dobbymoodge/env-wc-docs

<documentation> Add documentation for ENV: and ENV:* cartridge event subscriptions