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 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