Describe how sourmash handles two equivalent matches

[kternus]

This question comes up a lot, so it would be good to provide a brief explanation (or citation) in dahak about how sourmash gather handles two equivalent matches. We could describe how the process is different for sourmash LCA gather vs. regular sourmash gather.

@ctb pointed to this for reference - sourmash-bio/sourmash#390 (comment)