Update package info for HNSW

According to PR #608 #629, the HNSW package had been made hierarchical for Lucene 9.1.0, so it's not single-layer anymore as per the package info description:

https://github.com/apache/lucene/blob/248f067d5207da653f94751eb5d4e8a42ae1238e/lucene/core/src/java/org/apache/lucene/util/hnsw/package-info.java#L17-L22

It would be beneficial to consider updating the Javadoc pages from version 9.1.0 up to the latest (9.9.1) to reflect this change. This could be achieved by merging the commit(s) into those earlier version branches, if applicable.

* https://lucene.apache.org/core/9_1_0/core/org/apache/lucene/util/hnsw/package-summary.html
* https://lucene.apache.org/core/9_9_1/core/org/apache/lucene/util/hnsw/package-summary.html

Start with lucene/core/src/java/org/apache/lucene/util/hnsw/package-info.java, especially the lines linked in the issue, and compare the 9.1.0 and 9.9.1 Javadoc package-summary pages. Confirm which earlier version branches are in scope and update the package information so it reflects hierarchical HNSW rather than a single layer; done means the relevant Javadoc pages show the corrected description.

Written by the indexing model from the issue text.