Trivial: Fix validation comments

- Move comment about transaction/block weight calculation so it applies not only to the GetBlockWeight function but also to GetTransactionWeight
- Fix comment in validation.cpp referencing future deployment of BIP113. It has already been deployed.
- The doc comment for BLOCK_DOWNLOAD_WINDOW wasn't updated since pruning was introduced, so it still refers to pruning as something that might happen in the future. A larger BLOCK_DOWNLOAD_WINDOW window would now, indeed, make pruning harder.

src/validation.cpp

@@ -219,7 +219,7 @@ bool CheckFinalTx(const CTransaction &tx, int flags)
     // IsFinalTx() with one more than chainActive.Height().
     const int nBlockHeight = chainActive.Height() + 1;
 
-    // BIP113 will require that time-locked transactions have nLockTime set to
+    // BIP113 requires that time-locked transactions have nLockTime set to
     // less than the median time of the previous block they're contained in.
     // When the next block is created its previous block will be the current
     // chain tip, so we use that to calculate the median time passed to