LUCENE-10575: Fix some visibility issues

[romseygeek]

• ExternalRefSorter.ByteSequenceIterator should be public, as it's designed for external use
• MatchRegionRetriever refers to a private class in its public javadoc. I've just removed the link for now, but we could think about making the private implementation public as it looks as though it may be useful for other implementations of the highlighter?
• STMergingTermsEnum is now package-private. It uses private classes in some protected methods, and just making the whole thing package-private seemed to be the simplest solution to me. If we want to keep it public, then we should probably go through this more carefully and work out what needs to be available for extension and what doesn't.

[dweiss]

LGTM.

[romseygeek]

Thanks both!