rpc: Clarify that block count means height excl genesis

Summary:
Description of PR:

There is a common misconception that the block count returned by the blockchain rpcs includes the genesis block.
However, it really returns the height, which is 0 for the genesis block.
So clarify that and also remove the misleading "longest blockchain" comment.
Finally, fix the wallet test that incorrectly used this rpc.

Backport of Core PR16325

Test Plan:

ninja && ninja check-all
src/bitcoin-cli help getbestblockhash
src/bitcoin-cli help getbockcount

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7822