doc: add release notes

doc/release-notes-31384.md

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+- Node and Mining
+
+---
+
+- **PR #31384** fixed an issue where block reserved weight for block header, transactions count,
+ and coinbase transaction was done in two separate places. When the node constructs a new block,
+ e.g. a `getblocktemplate` RPC call doesn't generate the coinbase transaction. Instead, it reserves
+ `4,000` weight units (WU) for those data. Before this pull request, the default for `-blockmaxweight`
+ was `3,996,000`, The mining code 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,
+  `-blockreservedweight` (default: `8,000` weight units). This option specifies the weight units
+ reserved for the block header, transactions count, and coinbase transaction. The default value of
+ `-blockreservedweight` was chosen to preserve the previous behavior.
+
+- **Upgrade Note:** The default `-blockreservedweight` 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
+`-blockreservedweight` to `4,000` to maintain the previous behaviour.
+
+- Bitcoin Core will now **fail to start** if the `-blockmaxweight` or `-blockreservedweight` init parameter exceeds
+ the consensus limit of `4,000,000` weight units.
+- Bitcoin core will also **fail to start** when `-blockreservedweight` parameter value is lower than `2000` weight units.