doc: add release notes

doc/release-notes-31384.md

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+- Node and Mining
+
+---
+
+- **PR #31384** fixed an issue where the coinbase transaction weight was being reserved in two separate places.
+  When the node constructs a new block, e.g. for a `getblocktemplate` RPC call, it doesn't generate the coinbase
+  transaction. Instead it reserves `4,000` weight units (WU) for it. Before this pull request, the default for
+  `-blockmaxweight` was `3,996,000`, which effectively added another `4,000` weight units (WU) to this reservation,
+  leading to a template size of `3,992,000`.
+
+- The fix consolidates the reservation into a single place and introduces a new startup option,
+  `-coinbasereservedweight` (default: `8,000` weight units). This option specifies the weight units
+  reserved for the coinbase transaction. The default value of `-coinbasereservedweight` was chosen
+  to preserve the previous behavior.
+
+- **Upgrade Note:** The default `-coinbasereservedweight` ensures backward compatibility for users who did not override
+  `-blockmaxweight` and relied on the previous behavior.
+
+- Users who manually set `-blockmaxweight` to its maximum value `4,000,000` can lower
+`-coinbasereservedweight` to `4,000` to maintain the previous behaviour.
+
+- Bitcoin Core will now **fail to start** if the `-blockmaxweight` or `-coinbasereservedweight` init parameter exceeds
+  the consensus limit of `4,000,000` weight units.