Migrate pallet-elections-phragmen benchmark to v2 and improve doc (#6314)

Part of:

- #6202.

---------

Co-authored-by: Giuseppe Re <giuseppe.re@parity.io>
Co-authored-by: GitHub Action <action@github.com>

prdoc/pr_6314.prdoc

+title: Migrate pallet-elections-phragmen benchmark to v2 and improve doc
+doc:
+- audience: Runtime Dev
+  description: |-
+    Part of:
+
+    - #6202.
+crates:
+- name: pallet-elections-phragmen
+  bump: patch