doc: default minconf for getbalance should be 0
doc: default minconf for getbalance should be 0

Summary:
> - Default minconf for getbalance is 0 but example in doc was showing as 1.
> - at least 6 blocks confirmed now updated to be at least 6 confirmations to be more consistent with the terminology used elsewhere in the codebase and documentations.

This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/18502 | PR18502]]

Test Plan:
Check the new help message:

`src/bitcoin-cli help getbalance`

Check that the balance includes unconfirmed transactions by sending some fund to an external wallet and running immediately `src/bitcoin-cli getbalance`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8924

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