doc: Add missing top-level description to pruneblockchain RPC

Previously, the `pruneblockchain` RPC help output included only the method signature and arguments, with no top-level description explaining its purpose or constraints.

This PR adds a concise top-level description, improving documentation consistency and alerting users to the potential impacts of using the command.

Author: nervana21

src/rpc/blockchain.cpp

@@ -854,7 +854,9 @@ std::optional<int> GetPruneHeight(const BlockManager& blockman, const CChain& ch
 
 static RPCHelpMan pruneblockchain()
 {
-    return RPCHelpMan{"pruneblockchain", "",
+    return RPCHelpMan{"pruneblockchain",
+                "Attempts to delete block and undo data up to a specified height or timestamp, if eligible for pruning.\n"
+                "Requires `-prune` to be enabled at startup. While pruned data may be re-fetched in some cases (e.g., via `getblockfrompeer`), local deletion is irreversible.\n",
                 {
                     {"height", RPCArg::Type::NUM, RPCArg::Optional::NO, "The block height to prune up to. May be set to a discrete height, or to a " + UNIX_EPOCH_TIME + "\n"
             "                  to prune blocks whose block time is at least 2 hours older than the provided timestamp."},