doc: explain why we substract 4000 from DEFAULT_BLOCK_MAX_WEIGHT

src/policy/policy.h

@@ -19,7 +19,9 @@ class CCoinsViewCache;
 class CFeeRate;
 class CScript;
 
-/** Default for -blockmaxweight, which controls the range of block weights the mining code will create **/
+/** Default value for -blockmaxweight, which controls the range of block weights the mining code will create.
+ *  The 4000 weight units subtracted are reserved for the miner's coinbase transaction weight.
+**/
 static constexpr unsigned int DEFAULT_BLOCK_MAX_WEIGHT{MAX_BLOCK_WEIGHT - 4000};
 /** Default for -blockmintxfee, which sets the minimum feerate for a transaction in blocks created by mining code **/
 static constexpr unsigned int DEFAULT_BLOCK_MIN_TX_FEE{1000};