Cleanups for walletdir PR

Summary:
This is a followup for PR11466 backport.
It addresses a few nits and update the files.md doc.

Backport of core PR11726

Test Plan:

make check
./test/functional/test_runner.py wallet_multiwallet

Read files.md

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2395