[prdoc] Start BEEFY gadget by default for Polkadot nodes (#1945)

Just adding a prdoc for
https://github.com/paritytech/polkadot-sdk/pull/1913

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

prdoc/pr_1913.prdoc

+title: BEEFY on Polkadot
+
+doc:
+  - audience: Node Operator
+    description: |
+      The BEEFY gadget has been enabled on Polkadot by default.
+      If experiencing stability issues caused by BEEFY, it can be disabled using `--no-beefy` flag.
+      BEEFY doesn't (yet) support warp sync. So, attempting to Warp sync as a validator will throw an error.
+
+migrations:
+  db: []
+
+  runtime: []
+
+crates:
+  - name: polkadot-cli
+  - name: polkadot-service
+
+host_functions: []