docs/policy-language: be more explicit about future keywords (#5106)
docs/policy-language: be more explicit about future keywords (#5106)

Before, we started using `if` and `contains` rather casually. Adding a few
paragraphs to point out what those keywords are and how do without them.

Signed-off-by: Stephan Renatus <0d0a21c7604d0d452d5c1019ca18e48a97a9a057@gmail.com>
open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa