Merge #18410: Docs: Improve commenting for coins.cpp|h

21fa0a44ab [docs] use consistent naming for possible_overwrite (John Newbery)
2685c214cc [tests] small whitespace fixup (John Newbery)
e9936966c0 scripted-diff: Rename PRUNED to SPENT in coins tests (John Newbery)
c205979031 [docs] Improve commenting in coins.cpp|h (John Newbery)

Pull request description:

  - Add full commenting for spentness / DIRTYness / FRESHness and which combinations are valid
  - Remove the 'pruned' terminology, which doesn't make sense since per-txout chainstate db was merged (#10195).
  - Rename `potential_overwrite` to `possible_overwrite` to standardize terminology (there were previously examples of both, which made searching the codebase difficult).
  - Make other minor improvements to the comments

ACKs for top commit:
  jonatack:
    Re-ACK 21fa0a4 per `git diff 98bee55 21fa0a4` the only change since my previous review is the following code commenting diff in `src/coins.cpp::L177-179`;  rebuilt/ran unit tests anyway as a sanity check on the unit test changes.

Tree-SHA512: 391e01588ef5edb417250080cec17361f982c4454bc5f8c6d78bbd528c68a2bb94373297760691295c24660ce1022ad3ef7599762f736c8eed772ce096d38c3d

src/validation.cpp

@@ -1676,10 +1676,11 @@ int ApplyTxInUndo(Coin&& undo, CCoinsViewCache& view, const COutPoint& out)
             return DISCONNECT_FAILED; // adding output for transaction without known metadata
         }
     }
-    // The potential_overwrite parameter to AddCoin is only allowed to be false if we know for
-    // sure that the coin did not already exist in the cache. As we have queried for that above
-    // using HaveCoin, we don't need to guess. When fClean is false, a coin already existed and
-    // it is an overwrite.
+    // If the coin already exists as an unspent coin in the cache, then the
+    // possible_overwrite parameter to AddCoin must be set to true. We have
+    // already checked whether an unspent coin exists above using HaveCoin, so
+    // we don't need to guess. When fClean is false, an unspent coin already
+    // existed and it is an overwrite.
     view.AddCoin(out, std::move(undo), !fClean);
 
     return fClean ? DISCONNECT_OK : DISCONNECT_UNCLEAN;