miner: bugfix: fix duplicate weight reservation in block assembler

- This commit renamed coinbase_max_additional_weight to block_reserved_weight.

- Also clarify that the reservation is for block header, transaction count
  and coinbase transaction.

test/functional/mining_basic.py

@@ -26,6 +26,7 @@ from test_framework.messages import (
     CBlock,
     CBlockHeader,
     COIN,
+    DEFAULT_BLOCK_RESERVED_WEIGHT,
     ser_uint256,
 )
 from test_framework.p2p import P2PDataStore
@@ -77,7 +78,7 @@ class MiningTest(BitcoinTestFramework):
         mining_info = self.nodes[0].getmininginfo()
         assert_equal(mining_info['blocks'], 200)
         assert_equal(mining_info['currentblocktx'], 0)
-        assert_equal(mining_info['currentblockweight'], 4000)
+        assert_equal(mining_info['currentblockweight'], DEFAULT_BLOCK_RESERVED_WEIGHT)
 
         self.log.info('test blockversion')
         self.restart_node(0, extra_args=[f'-mocktime={t}', '-blockversion=1337'])