remove redundant strprintf and add missing sentence in doc

Summary:
This includes a sentence in the documentation of -stdin that was missed while backporting PR10997 (D1042),
and the removal of a redundant strprintf. The double space was already removed in D3058.

This is a backport of Core PR13905

Test Plan:
ninja && ninja check
bitcoin-cli -help

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8105