Document composition function credentials

https://github.com/crossplane/crossplane/pull/5543 adds the ability to pass credentials to a composition function.

```yaml
apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
name: parent
spec:
compositeTypeRef:
apiVersion: nop.example.org/v1alpha1
kind: XNopResource
mode: Pipeline
pipeline:
- step: example
functionRef:
name: function-needs-a-secret
credentials:
- name: credentials-for-something
source: Secret
secretRef:
namespace: crossplane-system
name: super-secret
```