Merge #11886: Clarify getbalance meaning a tiny bit in response to questions.

Summary:
c79d73d Clarify getbalance meaning a tiny bit in response to questions. (Matt Corallo)

Pull request description:

Someone was asking why getbalance "*" was more "correct" than getbalance, which should rarely be true...spendzeroconfchange was the issue.

Tree-SHA512: 90201cad1acec5161aee469fb4c6d737a0eb90f8380ac93abf0e41e0f02d120afcc3e2e873e5096d3655bb63bbd16fe99e72452f308d72e69139c7f6bb2d745e

Backport of Core PR11886
https://github.com/bitcoin/bitcoin/pull/11886/

Test Plan:

make check

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

Reviewed By: deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3698