docs: Add a note about default functions #8719

Job	Run time
Rego PR checks	16s
Generate Code	1m 30s
Go Lint	2m 38s
Go compat build/test (ubuntu-22.04, 1.19)	2m 1s
Go compat build/test (macos-latest, 1.19)	2m 58s
Check Generated	30s
Go Test (linux)	4m 50s
Go Test (darwin)	6m 8s
Go Build (linux amd64)	3m 13s
Go Build (linux arm64)	3m 36s
Go Build (windows)	3m 56s
Go Build (darwin)	2m 28s
Go Race Detector	8m 30s
WASM	15s
docker image smoke test	1m 3s
smoke-test-binaries (ubuntu-22.04, opa_linux_amd64)	15s
smoke-test-binaries (ubuntu-22.04, opa_linux_amd64_static, disabled)	13s
smoke-test-binaries (macos-latest, opa_darwin_amd64)	49s
smoke-test-binaries (windows-latest, opa_windows_amd64.exe)	52s
	46m 1s

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