Update docs to include information on strict-builtin-errors

[tsandall]

In v0.25.0 we switched the built-in error handling behaviour to turn runtime errors into undefined so that policy evaluation doesn't halt unexpectedly. However, in some cases, users need/want to be able to see those runtime errors surfaced and so they need to enable "strict built-in errors". The mechanism for doing this differs depending on whether the integration is via HTTP, CLI, Go, etc.

We should include a section in the docs on that explains the rationale and shows how to enable strict built-in errors via different APIs.

Relevant: https://github.com/open-policy-agent/opa/releases/tag/v0.25.0