[doc] release note effective-feerate and effective-includes RPC results

Summary:

No release note for submitpackage because it is regtest-only.

Partial backport of core#26646:
https://github.com/bitcoin/bitcoin/pull/26646/commits/da484bc738eca47936a31a2e5ad663417e19f311

Depends on D16400.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16401