trivial: Fixed typos and cleaned up language

Summary:
Backport of core PR13255
https://github.com/bitcoin/bitcoin/pull/13255/files

Depends on D3912.

Test Plan: Read the docs.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3921