Merge bitcoin/bitcoin#23777: doc: follow-ups from #23365 (index backwards search)

e4a8d561ed doc: add explanations for assert in index and magic numbers in test (Martin Zumsande)

Pull request description:

  This adds two explanations suggested in the review of #23365,  that I didn't manage to address before that PR was merged:

  https://github.com/bitcoin/bitcoin/pull/23365#discussion_r763981042
  https://github.com/bitcoin/bitcoin/pull/23365#discussion_r763982639

ACKs for top commit:
  jnewbery:
    ACK e4a8d561ed

Tree-SHA512: 0500c8abb37bb3e3694463ad5e74b2e1483615ccf1d7529b0d5faa694652ada17d242dc7fda6d995733766c627d54178a2c8fa21a570cdf13292f64ff5425b56

src/index/base.cpp

@@ -96,6 +96,8 @@ bool BaseIndex::Init()
                     prune_violation = false;
                     break;
                 }
+                // block->pprev must exist at this point, since block_to_test is part of the chain
+                // and thus must be encountered when going backwards from the tip
                 assert(block->pprev);
                 block = block->pprev;
             }

test/functional/feature_blockfilterindex_prune.py

@@ -29,6 +29,8 @@ class FeatureBlockfilterindexPruneTest(BitcoinTestFramework):
 
         self.log.info("prune some blocks")
         pruneheight = self.nodes[0].pruneblockchain(400)
+        # the prune heights used here and below are magic numbers that are determined by the
+        # thresholds at which block files wrap, so they depend on disk serialization and default block file size.
         assert_equal(pruneheight, 248)
 
         self.log.info("check if we can access the tips blockfilter when we have pruned some blocks")