Improve block validity/ConnectBlock() comments

Previously didn't make clear that the ContextualCheckBlock* functions
meant the block headers as context - not the UTXO set itself - and that
ConnectBlock() also did UTXO-related validity checks (in the future we
may split that functionality into a separate UTXO-specific contextual
check block function).

Also, reordered to put validity checks first for better readability.

src/main.h

@@ -395,23 +395,27 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus
 
 /** Functions for validating blocks and updating the block tree */
 
-/** Undo the effects of this block (with given index) on the UTXO set represented by coins.
- *  In case pfClean is provided, operation will try to be tolerant about errors, and *pfClean
- *  will be true if no problems were found. Otherwise, the return value will be false in case
- *  of problems. Note that in any case, coins may be modified. */
-bool DisconnectBlock(const CBlock& block, CValidationState& state, const CBlockIndex* pindex, CCoinsViewCache& coins, bool* pfClean = NULL);
-
-/** Apply the effects of this block (with given index) on the UTXO set represented by coins */
-bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pindex, CCoinsViewCache& coins, bool fJustCheck = false);
-
 /** Context-independent validity checks */
 bool CheckBlockHeader(const CBlockHeader& block, CValidationState& state, bool fCheckPOW = true);
 bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW = true, bool fCheckMerkleRoot = true);
 
-/** Context-dependent validity checks */
+/** Context-dependent validity checks.
+ *  By "context", we mean only the previous block headers, but not the UTXO
+ *  set; UTXO-related validity checks are done in ConnectBlock(). */
 bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, CBlockIndex *pindexPrev);
 bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIndex *pindexPrev);
 
+/** Apply the effects of this block (with given index) on the UTXO set represented by coins.
+ *  Validity checks that depend on the UTXO set are also done; ConnectBlock()
+ *  can fail if those validity checks fail (among other reasons). */
+bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pindex, CCoinsViewCache& coins, bool fJustCheck = false);
+
+/** Undo the effects of this block (with given index) on the UTXO set represented by coins.
+ *  In case pfClean is provided, operation will try to be tolerant about errors, and *pfClean
+ *  will be true if no problems were found. Otherwise, the return value will be false in case
+ *  of problems. Note that in any case, coins may be modified. */
+bool DisconnectBlock(const CBlock& block, CValidationState& state, const CBlockIndex* pindex, CCoinsViewCache& coins, bool* pfClean = NULL);
+
 /** Check a block is completely valid from start to finish (only works on top of our current best block, with cs_main held) */
 bool TestBlockValidity(CValidationState& state, const CChainParams& chainparams, const CBlock& block, CBlockIndex* pindexPrev, bool fCheckPOW = true, bool fCheckMerkleRoot = true);