Document WriteBatchWithIndex prefix iterator behavior

[richinmrudul]

Summary:
Documents that WriteBatchWithIndex::NewIteratorWithBase cannot infer the ReadOptions or prefix extractor used to create the base iterator. In particular, prefix_same_as_start on the base iterator is not applied to keys from the write batch, so callers that need prefix-bounded iteration must enforce the prefix boundary themselves.

Adds a focused test showing that a plain DB iterator respects prefix_same_as_start while the merged WriteBatchWithIndex iterator can still return in-batch keys outside the seek prefix.

Refs #14610

Test Plan:
make write_batch_with_index_test -j8
./write_batch_with_index_test --gtest_filter='Prefix'
git diff --check
make check-sources