Improve documentation of flexible controllers

[soren-da]

The original documentation on flexible controllers got removed/rewritten as every choice is now using flexible controllers, but the ability to use a party from a choice argument as a controller and its security implications are not documented.

• Document "flexible" controllers where a controller can be a party from an argument and the motivation/use-cases for doing this based on the blogpost.
• Remove the warning from the current specification of controllers with regard to visibility (not true anymore given contract disclosure).
• Highlight the security implications of opening up choices to arbitrary controllers, in particular if there are no further guards in place, in combination with explicit disclosure and interfaces, where a disclosed contract with "open" controllers anyone can exercise a choice and with interfaces the guards in a choice's implementation can change.

[basvangijzel-DA]

This sounds like a pure documentation issue. @carrielaben-da

Carrie, please ask Soren or Daml language if you need more details.