Merge #15267: doc: explain AcceptToMemoryPoolWorker's coins_to_uncache

5d26205272 doc: explain AcceptToMemoryPoolWorker's coins_to_uncache (James O'Beirne)

Pull request description:

  I found ATMPW's `coins_to_uncache` a little hard to understand (see #15264). This adds some doc for posterity.

ACKs for commit 5d2620:
  jnewbery:
    ACK 5d26205272

Tree-SHA512: 088508fa78012fab8680663c4e30f5cee29768416c2ca8b8b2abc29b6ac7067c5a589674f0254474a7ccc95477889d41719760f5796792bf492f51b3dd499c6c

src/coins.h

@@ -294,6 +294,10 @@ public:
     bool HaveInputs(const CTransaction& tx) const;
 
 private:
+    /**
+     * @note this is marked const, but may actually append to `cacheCoins`, increasing
+     * memory usage.
+     */
     CCoinsMap::iterator FetchCoin(const COutPoint &outpoint) const;
 };