test/doc: remove misleading comment and improve tests

remove misleading comment for m_blocks_unlinked since for
pruned nodes:
- usually A is the missing data (just like in non-pruned nodes)
- in PruneOneBlockFile, we remove entries once data for B is missing.

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

src/node/blockstorage.h

@@ -352,7 +352,6 @@ public:
 
     /**
      * All pairs A->B, where A (or one of its ancestors) misses transactions, but B has transactions.
-     * Pruned nodes may have entries where B is missing data.
      */
     std::multimap<CBlockIndex*, CBlockIndex*> m_blocks_unlinked;