Improve documentation for block coinbase transaction check

[teor2345]

Zcash has multiple coinbase transaction rules:

• one coinbase transaction, at the start of the transactions in a block Validate that only the first transaction in a block is a coinbase transaction  #298
• block coinbase height is one more than previous block Tracking: block header validation #477
• allowed coinbase transaction outputs ZIP-213: Validate Shielded Coinbase Outputs like other shielded outputs #608

Let's:

• rename coinbase_check to coinbase_is_first_check
• quote from the relevant part of the spec in the function documentation: https://github.com/ZcashFoundation/zebra/blob/main/zebra-consensus/src/verify/block.rs#L53