fix: update rest info on block size and json

Summary:

Addressing the ambiguous block size text in rest docs: PR18703

Also makes sure to let developers know there is .json option for the rest output format.

This is a backport of Core PR18810

Test Plan: Proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9064