Updating docs to reflect changes made here: dev/release#1 (comment 2843)

ref:
https://github.com/civicrm/civicrm-dev-docs/pull/301#issuecomment-324795431

Originally there were different types of extensions - payment processors, reports & modules. For a long time only modules have been recommended and I feel documenting the others only adds confusion.

mark `<requires>` and `<ext>` as experimental, and link to the PR which
makes them functional.

- `<classloader>`
- `<psr4>`
- `<requires>`
- `<ext>`

I don't know what these do, so I just added them with "not yet
documented"

This is already here:
https://docs.civicrm.org/dev/en/latest/extensions/#extension-names

This is already here:
https://docs.civicrm.org/dev/en/latest/extensions/publish/#prerequisites

https://wiki.civicrm.org/confluence/display/CRMDOC/Extension+Reference

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