Fix wrong version in clang-format error message and update the doc

Summary:
This fixes the error message that mistakenly request clang-format
version 7.x while D4539 introduced version >= 8.x.

Also update the doc as suggested in D4539.

Test Plan:

arc lint --everything

Read the doc.

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4551