refactor: Detach wallet transaction methods (followup for move-only)

Followup to commit "MOVEONLY: CWallet transaction code out of
wallet.cpp/.h" that detaches and renames some CWalletTx methods, making
into them into standalone functions or CWallet methods instead.

There are no changes in behavior and no code changes that aren't purely
mechanical. It just gives spend and receive functions more consistent
names and removes the circular dependencies added by the earlier
MOVEONLY commit.

There are also no comment or documentation changes. Removed comments
from transaction.h are just migrated to spend.h, receive.h, and
wallet.h.

test/functional/wallet_basic.py

@@ -600,7 +600,7 @@ class WalletTest(BitcoinTestFramework):
         total_txs = len(self.nodes[0].listtransactions("*", 99999))
 
         # Try with walletrejectlongchains
-        # Double chain limit but require combining inputs, so we pass SelectCoinsMinConf
+        # Double chain limit but require combining inputs, so we pass AttemptSelection
         self.stop_node(0)
         extra_args = ["-walletrejectlongchains", "-limitancestorcount=" + str(2 * chainlimit)]
         self.start_node(0, extra_args=extra_args)