Privatize CWallet::AddToWalletIfInvolvingMe

Summary:
And document in the header.

Partial backport of Core PR13651
https://github.com/bitcoin/bitcoin/pull/13651/commits/17e6aa8e333dbf3631fb09dc6eb7ede1937269df

Depends on D4675

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4676