Document realtime composition -> beta

[negz]

crossplane/crossplane#6407 promotes realtime composition to beta, on by default.

We need to update the docs to:

• Say that it's beta, not alpha
• Make sure the --poll-interval flag doesn't explicitly say it applies to XRs
• Document what things will trigger a reconcile?
• How does function TTL affect XR reconciliation?
• EnvironmentConfigs and ExtraResources are not included in the RTC watched resources