doc: add explanations for assert in index and magic numbers in test

These were suggested in review of #23365

Co-authored-by: John Newbery <john@johnnewbery.com>

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")