AcceptToMemoryPool: Re-add missing code comment

The comment on `view.SetBackend(dummy)` was removed when we backported
upstream locking PRs in zcash#5017. The upstream commit in
question removed a locking scope but did not remove the reference to
that scope in the comment. Our backport removed the outdated comment,
but should have modified it instead, because otherwise the existence of
`view.SetBackend(dummy)` is very confusing (as it disconnects the cache
from `pcoinsTip`, on the assumption that everything we need from it has
been cached via calls to `CCoinsViewCache::HaveCoins` and
`CCoinsViewCache::HaveShieldedRequirements`).

src/main.cpp

@@ -1892,6 +1892,7 @@ bool AcceptToMemoryPool(
 
         nValueIn = view.GetValueIn(tx);
 
+        // we have all inputs cached now, so switch back to dummy
         view.SetBackend(dummy);
 
         // Check for non-standard pay-to-script-hash in inputs