Migrate pallet-election-provider-multi-phase benchmark to v2 and improve doc (#6316)

Part of:

- #6202.

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Guillaume Thiolliere <guillaume.thiolliere@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/pr_6316.prdoc

+title: Migrate pallet-election-provider-multi-phase benchmark to v2 and improve doc
+doc:
+- audience: Runtime Dev
+  description: |-
+    Migrate pallet-election-provider-multi-phase benchmark to v2 and improve doc
+crates:
+- name: pallet-election-provider-multi-phase
+  bump: patch